Clarified --rollback command line option.

README.md

@@ -47,8 +47,7 @@ optional arguments:
                         corresponding to the private key file. The private key
                         file argument is required if this argument is
                         specified.
-  -b ROLLBACK, --rollback ROLLBACK
-                        Revert configuration <ROLLBACK> number of checkpoints.
+  -b N, --rollback N    Revert configuration N number of checkpoints.
   -k, --revoke          Revoke a certificate.
   -v, --view-checkpoints
                         View checkpoints and associated configuration changes.

letsencrypt/scripts/main.py

@@ -37,9 +37,8 @@ def main():
                              "private key file argument is required if this "
                              "argument is specified.")
     parser.add_argument("-b", "--rollback", dest="rollback", type=int,
-                        default=0,
-                        help="Revert configuration <ROLLBACK> number of "
-                             "checkpoints.")
+                        default=0, metavar="N",
+                        help="Revert configuration N number of checkpoints.")
     parser.add_argument("-k", "--revoke", dest="revoke", action="store_true",
                         help="Revoke a certificate.")
     parser.add_argument("-v", "--view-checkpoints", dest="view_checkpoints",