doc: change git clone url

The git clone url should be a Gerrit host and not Github, otherwise the
codereview command will fail.

	git-codereview: failed to load Gerrit origin: git origin must be a Gerrit host, not GitHub: https://github.com/golang/go

Change-Id: I62f62c86ee6dce0720a844fc56340135dfae8405
Reviewed-on: https://go-review.googlesource.com/117178Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/contribute.html

@@ -384,10 +384,10 @@ This is an overview of the overall process:
 
 <ul>
 <li>
-<b>Step 1:</b> Clone the Go source code from GitHub or go.googlesource.com
+<b>Step 1:</b> Clone the Go source code from go.googlesource.com
 and make sure it's stable by compiling and testing it once:
 <pre>
-$ git clone https://github.com/golang/go    # or https://go.googlesource.com/go
+$ git clone https://go.googlesource.com/go
 $ cd go/src
 $ ./all.bash                                # compile and test
 </pre>