docs: Fix docstrings to proper .rst syntax

Prevents compile errors when creating the user documentation.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

offlineimap/folder/Base.py

@@ -273,6 +273,7 @@ class BaseFolder(object):
         """Change the message from existing uid to new_uid
 
         If the backend supports it (IMAP does not).
+
         :param new_uid: (optional) If given, the old UID will be changed
             to a new UID. This allows backends efficient renaming of
             messages if the UID has changed."""