Remove uneeded wording in Dockerfile format

Docker-DCO-1.1-Signed-off-by: Brian Goff <cpuguy83@gmail.com> (github: cpuguy83)

docs/sources/reference/builder.rst

@@ -74,7 +74,7 @@ When you're done with your build, you're ready to look into
 2. Format
 =========
 
-The Dockerfile format is quite simple:
+Here is the format of the Dockerfile:
 
 ::