Fix directive example to match description

The description claims the directive is appearing after a comment but
the sample Dockerfile has the directive appear after an instruction.
Changed the ordering of the lines to match the example's description.

Signed-off-by: Remy Suen <remy.suen@gmail.com>

docs/reference/builder.md

@@ -199,8 +199,8 @@ directive:
 
 ```Dockerfile
 # About my dockerfile
-FROM ImageName
 # directive=value
+FROM ImageName
 ```
 
 The unknown directive is treated as a comment due to not being recognized. In