typing: fix a syntax error in mercurial/cext/bdiff.pyi

I noticed because `merge-pyi` on the non-cext implementations with this file as
input skipped the return type for this.  `pytype-single --parse-pyi` confirmed
it was a problem.

mercurial/cext/bdiff.pyi

@@ -5,7 +5,7 b' from typing import ('
 
 version: int
 
-def bdiff(a: bytes, b: bytes): bytes
+def bdiff(a: bytes, b: bytes) -> bytes: ...
 def blocks(a: bytes, b: bytes) -> List[Tuple[int, int, int, int]]: ...
 def fixws(s: bytes, allws: bool) -> bytes: ...
 def splitnewlines(text: bytes) -> List[bytes]: ...