minor usage update

README.rst

@@ -86,7 +86,7 @@ You normally will like to specify some "version record files"; see below.
 JSON logging
 ------------
 With ``--logger=json`` or ``--logger=both``, you can get a structured logging
-for programmatically consuming. You can use ``--json-log-fd=N`` to specify the
+for programmatically consuming. You can use ``--json-log-fd=FD`` to specify the
 file descriptor to send logs to (take care to do line buffering). The logging
 level option (``-l`` or ``--logging``) doesn't take effect with this.
 

nvchecker/core.py

@@ -26,7 +26,7 @@ def add_common_arguments(parser):
   parser.add_argument('--logger', default='pretty',
                       choices=['pretty', 'json', 'both'],
                       help='select which logger to use')
-  parser.add_argument('--json-log-fd',
+  parser.add_argument('--json-log-fd', metavar='FD',
                       type=lambda fd: os.fdopen(int(fd), mode='w'),
                       help='specify fd to send json logs to. stdout by default')
   parser.add_argument('-V', '--version', action='store_true',