[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:

• 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
• I have made corresponding changes to the documentation