Fix doc for `add_subparsers` arguments (GH-8884)


There was a missing comma.
(cherry picked from commit cc18258daf2727a4b8d657aaf0bf8a9cb0b54bb3)

Co-authored-by: Anthony Sottile <asottile@umich.edu>
diff --git a/Doc/library/argparse.rst b/Doc/library/argparse.rst
index 260e5cb..1973afc 100644
--- a/Doc/library/argparse.rst
+++ b/Doc/library/argparse.rst
@@ -1539,7 +1539,7 @@
 
 .. method:: ArgumentParser.add_subparsers([title], [description], [prog], \
                                           [parser_class], [action], \
-                                          [option_string], [dest], [required] \
+                                          [option_string], [dest], [required], \
                                           [help], [metavar])
 
    Many programs split up their functionality into a number of sub-commands,