flag: removed the colon after "Usage" in the documentation

Removing the colon will make the "Usage" and "Command line flag syntax" in the docs, a header when interpreted by godoc.

Fixes #25749

Change-Id: Ifc5572e171db1aaef9775b1d6c86091a8f2528fd
GitHub-Last-Rev: 1b579734308cbcb96f17d945b9c7af70e259cb5b
GitHub-Pull-Request: golang/go#25750
Reviewed-on: https://go-review.googlesource.com/116555Reviewed-by: Rob Pike <r@golang.org>

src/flag/flag.go

@@ -5,7 +5,7 @@
 /*
 	Package flag implements command-line flag parsing.
 
-	Usage:
+	Usage
 
 	Define flags using flag.String(), Bool(), Int(), etc.
 
@@ -35,7 +35,7 @@
 	slice flag.Args() or individually as flag.Arg(i).
 	The arguments are indexed from 0 through flag.NArg()-1.
 
-	Command line flag syntax:
+	Command line flag syntax
 		-flag
 		-flag=x
 		-flag x  // non-boolean flags only