Issue #13540: add missing markup.

Doc/library/argparse.rst

@@ -1237,8 +1237,8 @@ which processes arguments from the command-line. Any object which follows
 this API may be passed as the ``action`` parameter to
 :meth:`add_argument`.
 
-.. class:: Action(option_strings, dest, nargs=None, const=None, default=None,
-                  type=None, choices=None, required=False, help=None,
+.. class:: Action(option_strings, dest, nargs=None, const=None, default=None, \
+                  type=None, choices=None, required=False, help=None, \
                   metavar=None)
 
 Action objects are used by an ArgumentParser to represent the information