Change Python-as-executable-script example to use "#! /usr/bin/env python"

Change Python-as-executable-script example to use "#! /usr/bin/env python"
since that's now the recommended way to do it.

In pickling discussion, change "code{pickle}" (not missing leading "\") to
"This" since the immediately preceeding sentence ended with the same text
(with the proper "\").  Fixes a formatting bug and an odd glitch in the
writing.