concurrent.futures: Fix typo in docstring (GH-92121)

(cherry picked from commit b11243e)

Co-authored-by: Yiannis Hadjicharalambous <hadjicharalambous.yiannis@gmail.com>

Author: miss-islington

Lib/concurrent/futures/_base.py

@@ -381,7 +381,7 @@ def running(self):
             return self._state == RUNNING
 
     def done(self):
-        """Return True of the future was cancelled or finished executing."""
+        """Return True if the future was cancelled or finished executing."""
         with self._condition:
             return self._state in [CANCELLED, CANCELLED_AND_NOTIFIED, FINISHED]