CONTRIBUTING.md: update the command.

Update the command to `go get -u golang.org/x/lint/golint` to make it consistent with `README.md'.

Change-Id: I0e20881d34a9689577d9c9f5ad74008782695c95
GitHub-Last-Rev: 096da70784bbea3fe0cd7475f857b3dd97474a30
GitHub-Pull-Request: golang/lint#398
Reviewed-on: https://go-review.googlesource.com/108995
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 971da12..1fadda6 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -6,7 +6,7 @@
 
 Check you have the latest version of its dependencies. Run
 ```
-go get -u github.com/golang/lint
+go get -u golang.org/x/lint/golint
 ```
 If you still have problems, consider searching for existing issues before filing a new issue.