Fix git.Commit docs typo

Author: reuben-sutton

git/objects/commit.py

@@ -93,7 +93,7 @@ def __init__(self, repo, binsha, tree=None, author=None, authored_date=None, aut
             is the committed DateTime - use time.gmtime() to convert it into a
             different format
         :param committer_tz_offset: int_seconds_west_of_utc
-            is the timezone that the authored_date is in
+            is the timezone that the committed_date is in
         :param message: string
             is the commit message
         :param encoding: string