Update commit.py constructor docstring

Fixes issue gitpython-developers#806: Commit requires author parameter to be of Actor type, not string.

Author: CptMikhailov

Diff for: git/objects/commit.py

@@ -81,7 +81,7 @@ def __init__(self, repo, binsha, tree=None, author=None, authored_date=None, aut
         :param tree: Tree
             Tree object
         :param author: Actor
-            is the author string ( will be implicitly converted into an Actor object )
+            is the author Actor object
         :param authored_date: int_seconds_since_epoch
             is the authored DateTime - use time.gmtime() to convert it into a
             different format