fix tolerance in read pixels tests

This was accidentally loosened up

Change-Id: I7a95ed5b56aff06114ceaa9c2a3719bd85d4add4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/252934
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
1 file changed