docs: Very minor correction

[neel-maniar]

A very minor correction to the docs/examples/intro-to-gps.py document. (It is a great introduction to GPs, thank you for including it!)

In line 406, it says $\mathbf{X}\in\mathbf{X}$ whereas it should say $\mathbf{X}\in\mathcal{X}$.

[st--]

Hi @neel-maniar, thanks for noticing! would you be up for making the change yourself directly and preparing a pull request? :)

[neel-maniar]

Hi @st--, thanks for your prompt reply! I am a bit new to GitHub, but have attempted to make a pull request. Hopefully it works as expected!

[thomaspinder]

Resolved in #433