[NFC] Remove reference to Differential. (llvm#101587)

Remove reference to Differential since Phabricator is not used
anymore.

llvm/docs/Contributing.rst

@@ -58,7 +58,7 @@ Once you have a patch ready, it is time to submit it. The patch should:
 * conform to the :doc:`CodingStandards`. You can use the `clang-format-diff.py`_ or `git-clang-format`_ tools to automatically format your patch properly.
 * not contain any unrelated changes
 * be an isolated change. Independent changes should be submitted as separate patches as this makes reviewing easier.
-* have a single commit (unless stacked on another Differential), up-to-date with the upstream ``origin/main`` branch, and don't have merges.
+* have a single commit, up-to-date with the upstream ``origin/main`` branch, and don't have merges.
 
 .. _format patches: