Add ".py" suffix to sphinx-argparse generated docs

This is important to produce the right command syntax when generating
command line reference in the user manual.  However, we shouldn't add
this kludge to the individual programs, but only to the documentation
using the :prog: syntax.

Change-Id: I2ec7ab00c63d5d386f187e54755c71ffc2dce429
diff --git a/docs/legacy.rst b/docs/legacy.rst
index d93358f..0961d17 100644
--- a/docs/legacy.rst
+++ b/docs/legacy.rst
@@ -98,3 +98,4 @@
 .. argparse::
    :module: pySim-read
    :func: option_parser
+   :prog: pySim-read.py