Update build doc

docs/sources/commandline/command/build.rst

@@ -2,9 +2,9 @@
 :description: Build a new image from the Dockerfile passed via stdin
 :keywords: build, docker, container, documentation
 
-========================================================
-``build`` -- Build a container from Dockerfile via stdin
-========================================================
+================================================
+``build`` -- Build a container from a Dockerfile
+================================================
 
 ::