Fix typo client_label -> client-label for strings configuration files

I suspect the other underscore containing settings have the same typo - but I hadn't have time to check it, so I'm leaving it as a suggestion for further work. Sorry 'bout that.

doc/clients.rst

@@ -234,7 +234,7 @@ wait_timeout
      connection to be established before failing with a
      ``ZEO.Exceptions.ClientDisconnected`` exception.
 
-client_label
+client-label
    A short string to display in *server* logs for an event relating to
    this client. This can be helpful when debugging.