update example docstring of afm.py

[ImportanceOfBeingErnest]

In order for the example to work in python 3, the file needs to be read in binary mode: open(afm_fname, 'rb')

Issue description can be found here: https://stackoverflow.com/questions/48793711/calculating-the-width-height-of-strings-on-python

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is PEP 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[timhoffm]

Sorry for the noise. I was accidentally on an older branch.