Fix documentation typo (#948)

Author: aaqibb13

docs/server.rst

@@ -325,7 +325,7 @@ that belong to a namespace can be created as methods of a subclass of
 
     sio.register_namespace(MyCustomNamespace('/test'))
 
-For asyncio based severs, namespaces must inherit from
+For asyncio based servers, namespaces must inherit from
 :class:`socketio.AsyncNamespace`, and can define event handlers as coroutines
 if desired::