Update iOS build instructions for new simulator arg.
Update iOS build instructions for new simulator arg.

Bug: skia:12880
Change-Id: Ia91c02dce5d994ebe169347e1e6fbe4a72bffa6f
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/529119
Reviewed-by: Ben Wagner <53965154ffeecc4a10886eab9259e95965f2979a@google.com>
Commit-Queue: Jim Van Verth <509afb0bfbb23ecaaf1fb3323209f531b45e75a6@google.com>

google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia