cmd/go: re-run mkdoc.sh

Adds $DOLLAR to doc.go

Change-Id: Ia0ccdd9b5292010297ae95debab4acf50d3c4d9c
Reviewed-on: https://go-review.googlesource.com/8600Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/doc.go

@@ -271,6 +271,8 @@ Go generate sets several variables when it runs the generator:
 		The base name of the file.
 	$GOPACKAGE
 		The name of the package of the file containing the directive.
+	$DOLLAR
+		A dollar sign.
 
 Other than variable substitution and quoted-string evaluation, no
 special processing such as "globbing" is performed on the command