standby promote: use variable name "local_conn" for the local connection handle

This is consistent with usage in other functions, and makes it easier to
differentiate between the local node connection and the primary connection.

repmgr-client.c

@@ -1953,6 +1953,7 @@ check_cli_parameters(const int action)
 	{
 		switch (action)
 		{
+			case STANDBY_PROMOTE:
 			case STANDBY_SWITCHOVER:
 				break;
 			default: