Update the website FAQ

The [FAQ](https://github.com/SasView/sasview.github.io/blob/master/faq.md) for sasview.org has Windows-specific instructions for dealing with installation issues, but more users are having issues installing on Macs. The FAQ item [https://www.sasview.org/faq/#why-will-sasview-no-longer-start](https://www.sasview.org/faq/#why-will-sasview-no-longer-start) could use some generalization.