[pyos] Broken link in docs

On https://python-graphblas.readthedocs.io/en/latest/user_guide/fundamentals.html, the graphblas.org link is broken, probably due to a double-`:` in `https:://graphblas.org`

<img width="449" alt="image" src="https://user-images.githubusercontent.com/1212871/223452528-fcabc80e-bea7-41a6-a2df-27ef16aedea5.png">

https://github.com/pyOpenSci/software-submission/issues/81