Improve warning message

cf. #302

lib/ridgepole/dsl_parser.rb

@@ -41,7 +41,7 @@ def check_foreign_key_without_index(table_name, attrs)
 
         Ridgepole::Logger.instance.warn(<<-MSG)
 [WARNING] Table `#{table_name}` has a foreign key on `#{fk_index}` column, but doesn't have any indexes on the column.
-          Although an index will be added automatically by InnoDB, please add an index explicitly for your future operations.
+          Although an index will be added automatically by InnoDB, please add an index explicitly before the next operation.
         MSG
       end
     end