doc: Fix the canonical location

Change-Id: I3030d678c4eee85970826c86332a2b818cf7f5b6
Reviewed-on: https://go-review.googlesource.com/1217Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

CONTRIBUTING.md

 The Go project is an open source project.
 For information, see: https://golang.org/
 
-Its canonical Git repository is located at https://go.googlecode.com/go.
+Its canonical Git repository is located at https://go.googlesource.com/go.
 (There is a mirror of the repository at https://github.com/golang/go.)
 
 Report issues here: https://github.com/golang/go/issues