pythongh-138019: improve help message for json.tool (python#138037)

Author: yihong0618

Lib/json/tool.py

@@ -46,7 +46,8 @@ def main():
                    'to validate and pretty-print JSON objects.')
     parser = argparse.ArgumentParser(description=description, color=True)
     parser.add_argument('infile', nargs='?',
-                        help='a JSON file to be validated or pretty-printed',
+                        help='a JSON file to be validated or pretty-printed; '
+                             'defaults to stdin',
                         default='-')
     parser.add_argument('outfile', nargs='?',
                         help='write the output of infile to outfile',