GL_NUM_REQUESTABLE_EXTENSIONS_ANGLE had a typo between the spec and the
header but any user who used our header would still get the correct
beaviour.

BUG=angleproject:3861

Change-Id: Ibc85607ad41a8419246e057065ab4ce797f0ce66
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1772376Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>