Fix description of argument

bin/tuttest

@@ -10,7 +10,7 @@ if __name__ == "__main__":
     parser = argparse.ArgumentParser(description='A tutorial tester script. Extract the code blocks from tutorial and see if, when followed, the tutorial actually works.')
 
     parser.add_argument('filename', metavar='filename', type=str, help='filename with tutorial')
-    parser.add_argument('commands', metavar='commands', nargs='?', type=str, help='optional names to give to the extracted snippets, provided as list')
+    parser.add_argument('commands', metavar='commands', nargs='?', type=str, help='optional comma-separated list of names of snippets to extract if you want a subset')
 
     parser.add_argument('--prefix-lines-with', metavar='prefix', type=str, help='string to prefix each command with')
     parser.add_argument('--single-command', action='store_true', help='executes all snippets in single command')