Update What_Is_a_Font.md

Change log:
1. Made a typo correction (ot to)
2. Added a para in "contrast" about choosing a consistent stroke width vs having contrast in your font design. Mention about slab-serifs and how they seem to use consistent stroke width than be just about serifs.
3. added sub-section Serfif>bracket, Serif>Slab-serif adn Serif>Serif Terminals.
4. Added to clarify counter vs bowls
5. ? want to but don't see any option to add illustrations for the above.