Fix argument name

Author: mmoya

prometheus_client/__init__.py

@@ -42,7 +42,7 @@ def register(self, collector):
     with self._lock:
       self._collectors.add(collector)
 
-  def unregister(self, metric):
+  def unregister(self, collector):
     '''Remove a collector from the registry.'''
     with self._lock:
       self._collectors.remove(collector)