CLEANUP: Fix a typo in the protocol header file

Fixes a typo in the code comments of the header file holding the general
protocol primitives.
diff --git a/include/proto/protocol.h b/include/proto/protocol.h
index 857f9e6..7bbebb8 100644
--- a/include/proto/protocol.h
+++ b/include/proto/protocol.h
@@ -35,7 +35,7 @@
  */
 void protocol_unregister(struct protocol *proto);
 
-/* binds all listeneres of all registered protocols. Returns a composition
+/* binds all listeners of all registered protocols. Returns a composition
  * of ERR_NONE, ERR_RETRYABLE, ERR_FATAL.
  */
 int protocol_bind_all(char *errmsg, int errlen);