Remove redundant comma in argparse HOWTO (#1141)

Reported by Sean Canavan on docs@p.o.
diff --git a/Doc/howto/argparse.rst b/Doc/howto/argparse.rst
index 7e161a5..9d770f5 100644
--- a/Doc/howto/argparse.rst
+++ b/Doc/howto/argparse.rst
@@ -221,7 +221,7 @@
 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