[Contributing] Fix "When you finish" => "When you are finished"

As per @xabbuh suggestion: #3421 (comment)

contributing/code/patches.rst

@@ -398,7 +398,7 @@ To squash all commits into the first one, remove the word ``pick`` before the
 second and the last commits, and replace it by the word ``squash`` or just
  ``s``. When you save, Git will start rebasing, and if successful, will ask
  you to edit the commit message, which by default is a listing of the commit
- messages of all the commits. When you finish, execute the push command.
+ messages of all the commits. When you are finished, execute the push command.
 
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