Fix docs: hhea.Descender should be negative for descent below the baseline

Refer to: https://glyphsapp.com/learn/vertical-metrics
hheaDescender: the depth of the descenders in units (negative value)

Author: TakWolf

docs/OpenTypeFeatureFileSpecification.md

@@ -3350,7 +3350,7 @@ For example:
 table hhea {
     CaretOffset -50;
     Ascender 800;
-    Descender 200;
+    Descender -200; # Note that Descender is negative for descent below the baseline.
     LineGap 200;
 } hhea;
 ```