manifest: update comment to be about bytearray

Looks like a leftover from 2a18e9e6ca43 (py3: use bytearray() instead
of array('c', ...) constructions, 2017-03-12).

mercurial/manifest.py

@@ -591,7 +591,7 b' class manifestdict(object):'
             return self._lm.text()
 
     def fastdelta(self, base, changes):
-        """Given a base manifest text as an array.array and a list of changes
+        """Given a base manifest text as a bytearray and a list of changes
         relative to that text, compute a delta that can be used by revlog.
         """
         delta = []