cmd/godoc: remove extra // characters from deprecation notice

The comment is already inside a /* */ block, so no need for //.

Updates golang/go#49212.

Change-Id: I6d50cedab363ed312d85e66797cbc4f652e56dab
Reviewed-on: https://go-review.googlesource.com/c/tools/+/367014
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Dmitri Shuralyov <dmitshur@golang.org>

cmd/godoc/doc.go

@@ -108,7 +108,7 @@ see https://golang.org/pkg/testing/#hdr-Examples for the conventions.
 See "Godoc: documenting Go code" for how to write good comments for godoc:
 https://golang.org/doc/articles/godoc_documenting_go_code.html
 
-// Deprecated: godoc cannot select what version of a package is displayed.
-// Instead, use golang.org/x/pkgsite/cmd/pkgsite.
+Deprecated: godoc cannot select what version of a package is displayed.
+Instead, use golang.org/x/pkgsite/cmd/pkgsite.
 */
 package main // import "golang.org/x/tools/cmd/godoc"