python-wrapper: Fix variable name for sed expr.

Regression introduced by 5f557885

.

The commit not only changes documentation, but also changed a few
variable names. One of them is $i which now is $f and it contains the
name of the file to wrap.

This was accidentally found by @Profpatsch (thanks!) who found himself
getting the basename of the last patch file to end up in sys.argv[0].
The reason for this is that $i is used in the for loop of the generic
patchPhase and thus is reused later when the Python file is to be
wrapped.

I have also added a small comment noting about this, to be sure that
this won't accidentally occur the next time someone changes variable
names.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>