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: 096da70
GitHub-Pull-Request: #398
Reviewed-on: https://go-review.googlesource.com/108995
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

Author: Wei Fu

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
 ```
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