flag: fix headers in documentation, again

Godoc expects text after a header, not code.

Change-Id: I99f412ad15e10bf9cea0dbd19019ed6ce477736c
Reviewed-on: https://go-review.googlesource.com/117595Reviewed-by: Rob Pike <r@golang.org>

src/flag/flag.go

@@ -36,6 +36,9 @@
 	The arguments are indexed from 0 through flag.NArg()-1.
 
 	Command line flag syntax
+
+	The following forms are permitted:
+
 		-flag
 		-flag=x
 		-flag x  // non-boolean flags only