tests: fix a typo on diff.py

Reviewed By: quark-zju

Differential Revision: D55380564

fbshipit-source-id: 79c498f0e0ae4a41577dd928feb0785882a5718a

eden/scm/sapling/testing/t/diff.py

@@ -199,7 +199,7 @@ def match(self, actual: str) -> bool:
         return self._matchandnormalizecached(actual)[0]
 
     def normalize(self, actual: str) -> Tuple[str, str]:
-        """Normalzie (actual, expected) for plain text diff.
+        """Normalize (actual, expected) for plain text diff.
 
         Return (actual, expected) pair, which is more friendly for a plain
         text diff algorithm.