Clarify .dockerignore example for Markdown files

The current documentation correctly states that dockerignore pattern
searches are non-recursive. However, the example given for Markdown
files seems to contradict this by saying that `*.md` will exclude *all*
Markdown files. This commit clarifies the situation by explicitly
specifying that `*.md` will only exclude files in the root directory of
the project.

Signed-off-by: Tim Wraight <tim.wraight@tangentlabs.co.uk>

docs/reference/builder.md

@@ -208,11 +208,11 @@ This file causes the following build behavior:
 
 | Rule           | Behavior                                                                                                                                                                     |
 |----------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
-| `*/temp*`      | Exclude all files with names starting with`temp` in any subdirectory below the root directory. For example, a file named`/somedir/temporary.txt` is ignored.              |
+| `*/temp*`      | Exclude all files with names starting with`temp` in any subdirectory below the root directory. For example, a file named`/somedir/temporary.txt` is ignored.                 |
 | `*/*/temp*`    | Exclude files starting with name `temp` from any subdirectory that is two levels below the root directory. For example, the file `/somedir/subdir/temporary.txt` is ignored. |
-| `temp?`        | Exclude the files that match the pattern in the root directory. For example, the files `tempa`, `tempb` in the root directory are ignored.                               |
-| `*.md `        | Exclude all markdown files.                                                                                                                                                  |
-| `!LICENSE.md` | Exception to the exclude all Markdown files is this file,  `LICENSE.md`, include this file in the build.                                                                     |
+| `temp?`        | Exclude the files that match the pattern in the root directory. For example, the files `tempa`, `tempb` in the root directory are ignored.                                   |
+| `*.md `        | Exclude all markdown files in the root directory.                                                                                                                            |
+| `!LICENSE.md`  | Exception to the Markdown files exclusion is this file,  `LICENSE.md`, Include this file in the build.                                                                       |
 
 The placement of  `!` exception rules influences the matching algorithm; the
 last line of the `.dockerignore` that matches a particular file determines
@@ -229,8 +229,9 @@ example:
 ```
 
 The build would exclude `LICENSE.md` because the last `*.md` rule adds all
-Markdown files back onto the ignore list. The `!LICENSE.md` rule has no effect
-because the subsequent `*.md` rule overrides it.
+Markdown files in the root directory back onto the ignore list. The
+`!LICENSE.md` rule has no effect because the subsequent `*.md` rule overrides
+it.
 
 You can even use the  `.dockerignore` file to ignore the `Dockerfile` and
 `.dockerignore` files. This is useful if you are copying files from the root of