[docs] hhea.Descender should be negative for descent below the baseline

[TakWolf]

Description

Refer to: https://glyphsapp.com/learn/vertical-metrics

hhea: hheaAscender: the height of the ascenders in units hheaDescender: the depth of the descenders in units (negative value) # <----- This hheaLineGap: the recommended whitespace between lines

Checklist:

• [x] I have followed the Contribution Guidelines
• [ ] I have added test code and data to prove that my code functions correctly
• [ ] I have verified that new and existing tests pass locally with my changes
• [ ] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation