Fix formatting of docstring

[habi]

I cannot really test, but according to http://docutils.sourceforge.net/docs/ref/rst/roles.html#subscript this should correct the display on https://matplotlib.org/api/_as_gen/matplotlib.axes.Axes.ticklabel_format.html

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 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

[anntzer]

Thanks.

[habi]

Very welcome!