[docs] Fix suggested darker command in coding standards (#69860)

Do not attempt to format deleted files.

Author: kuhar

llvm/docs/CodingStandards.rst

@@ -148,7 +148,7 @@ For example:
 
 .. code-block:: bash
 
-   $ darker -r HEAD^ $(git diff --name-only HEAD^)
+   $ darker -r HEAD^ $(git diff --name-only --diff-filter=d HEAD^)
 
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