Remove redundant comma in argparse HOWTO (GH-1141)

Reported by Sean Canavan on docs@p.o.

(cherry picked from commit 8526fb74)

Doc/howto/argparse.rst

@@ -223,7 +223,7 @@ before proceeding.
 Introducing Optional arguments
 ==============================
 
-So far we, have been playing with positional arguments. Let us
+So far we have been playing with positional arguments. Let us
 have a look on how to add optional ones::
 
    import argparse