Fix assertion error syntax.

dulwich/protocol.py

@@ -122,7 +122,7 @@ class Protocol(object):
             pkt_contents = read(size-4)
             if len(pkt_contents) + 4 != size:
                 raise AssertionError('Length of pkt read %04x does not match length prefix %04x.'
-                                     .format(len(pkt_contents) + 4, size))
+                                     % (len(pkt_contents) + 4, size))
             return pkt_contents
         except socket.error as e:
             raise GitProtocolError(e)