python: fix typo in Socket::__str__() function

The property name used in __str__ should be local_port
instead of localPort to get rid of the AttributeError.

>>> str(s)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../netlink/core.py", line 172, in __str__
    return 'nlsock<{0}>'.format(self.localPort)
AttributeError: 'Socket' object has no attribute 'localPort'

Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Thomas Graf <tgraf@suug.ch>

python/netlink/core.py

@@ -198,7 +198,7 @@ class Socket(object):
         capi.nl_socket_free(self._sock)
 
     def __str__(self):
-        return 'nlsock<{0}>'.format(self.localPort)
+        return 'nlsock<{0}>'.format(self.local_port)
 
     @property
     def local_port(self):