fix CONTRIBUTING.md PR test dependency

Fixes incorrect dependency line for testing PRs in CONTRIBUTING.md.

Author: lundibundi

CONTRIBUTING.md

@@ -40,7 +40,7 @@ Pull requests often need some real-world testing.
 1. In your `package.json`, change the line with `webpack-dev-server` to:
 
   ```json
-  "webpack-dev-server": "webpack/webpack-dev-server#<ID>/head"
+  "webpack-dev-server": "github:webpack/webpack-dev-server#pull/<ID>/head"
   ```
 
   `<ID>` is the ID of the pull request.