Fix SkFontDescriptor kSlant documentation

This change does not change any current behavior, since currently kSlant
is only checked for zero or non-zero. However, the documentation and use
should be updated to reflect reality. A slant of -10 is "normal" but a
request for slant should be -20.

Change-Id: I079088827ae01b2b7e33ec6836f7f3e41403ca16
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1205058
Reviewed-by: Florin Malita <fmalita@google.com>
Reviewed-by: Julia Lavrova <jlavrova@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>