Use US spelling.

Author: aaugustin

src/websockets/legacy/protocol.py

@@ -1372,7 +1372,7 @@ def fail_connection(
 
         This requires:
 
-        1. Stopping all processing of incoming data, which means cancelling
+        1. Stopping all processing of incoming data, which means canceling
            :attr:`transfer_data_task`. The close code will be 1006 unless a
            close frame was received earlier.