doc: renamed remote's name

To keep consistency through-out the guide.

Fixes: #26045

PR-URL: #26050
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Masashi Hirano <shisama07@gmail.com>

doc/guides/contributing/pull-requests.md

@@ -301,7 +301,7 @@ changes that have landed in `master` by using `git rebase`:
 
 ```text
 $ git fetch --all
-$ git rebase origin/master
+$ git rebase upstream/master
 $ git push --force-with-lease origin my-branch
 ```