Fix link to python file

[mwcraig]

See https://github.com/mwcraig/ccd-reduction-and-photometry-guide/pull/209#pullrequestreview-370062661

[morrme]

@mwcraig
Clicking on the image_sim.py link on https://github.com/mwcraig/ccd-reduction-and-photometry-guide/blob/master/notebooks/01-04-Nonuniform-sensitivity.ipynb

takes me directly to https://render.githubusercontent.com/view/image_sim.py .

What is the desired result?

[mwcraig]

@morrme -- thanks for reporting this. I think that is the intended behavior for now. As of last summer there wasn't a good way to link to a Python file from a notebook. That seemed odd to me and it may have been fixed.