Append .git to upstream URLs

The upstream URLs are missing the `.git` suffix. You can see this minor omission if you run the `git remote -v` command in your local `website` repo.

Author: rolfedh

content/en/docs/contribute/new-content/open-a-pr.md

@@ -123,8 +123,8 @@ Make sure you have [git](https://git-scm.com/book/en/v2/Getting-Started-Installi
     ```bash
     origin	[email protected]:<github_username>/website.git (fetch)
     origin	[email protected]:<github_username>/website.git (push)
-    upstream	https://github.com/kubernetes/website (fetch)
-    upstream	https://github.com/kubernetes/website (push)
+    upstream	https://github.com/kubernetes/website.git (fetch)
+    upstream	https://github.com/kubernetes/website.git (push)
     ```
 
 6. Fetch commits from your fork's `origin/master` and `kubernetes/website`'s `upstream/master`: