DOC: Spelling should "focusing" rather than "focussing" in README.rst of documentation

### Issue with current documentation:

From here: https://www.pymc.io/about/ecosystem.html

> [CausalPy](https://github.com/pymc-labs/CausalPy): A package focussing on causal inference in quasi-experimental settings.

### Idea or request for content:

Just correct the spelling. Maybe I should do it just to familiarize myself with PyMC's contribution process?