fix unpremul()
fix unpremul()

Just a typo.  Rewritten to make premul vs. unpremul stand out more.

Change-Id: I6db425a97a1333a6c5d2e867115c7f77f73d0482
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/280378
Auto-Submit: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>
Reviewed-by: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>
Commit-Queue: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>

google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia