CONTRIBUTING.md: fix link to codereview(1)

Change-Id: I259a397398776bf9a172f1f4ed69e608166f35fb
Reviewed-on: https://plan9port-review.googlesource.com/1097
Reviewed-by: Russ Cox <rsc@swtch.com>
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 54ca579..50b65d6 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -1,5 +1,5 @@
 Plan 9 from User Space uses Gerrit instead of GitHub pull requests.
-See the [codereview(1)](http://swtch.com/usr/local/plan9/man/man1/codereview.html) man page.
+See the [codereview(1)](http://swtch.com/plan9port/man/man1/codereview.html) man page.
 
 If you would like to use git directly instead of the codereview(1) wrapper, that is fine too.
 Follow the instructions on the RE2 [Contributing](https://github.com/google/re2/wiki/Contribute) wiki page,