Fix typos in Spitzer PSF photometry notebook

@adonath Thanks for this!  I have a couple of quick questions.  Where does your comparison catalog come from and how was its photometry generated?  I'm curious because of the apparent coordinate offset that you mention, and also there appears to be less scatter when comparing the catalog to `GaussianPSF` fitting.

Also, when fitting I get many repeated warnings:
```
WARNING: Model is linear in parameters; consider using linear fitting methods. [astropy.modeling.fitting]
```
Presumably this is because you are using the non-linear `LevMarLSQFitter()`.  Can a linear fitter be used instead?  If not, I think we should suppress the warning.