README: markdown tweaks and fixes

[luzpaz]

• Fixed typos
• Added hyplink to Dispersion example
• Italicized screenshot explanations
• Reformatted documentation a bit

Preview: https://github.com/luzpaz/OpticsWorkbench/blob/README-tweaks/README.md

[luzpaz]

I'm not proposing this is all more improved. Of course i'm very open to feedback and revision per @chbergmann