Update Skia expectation for bug_880920.pdf

CL [1] has fixed the issue that the dot density in the Skia rendering
result of bug_880920.pdf is way less than its AGG expectation.
Therefore update this corpus test's Skia expectation accordingly.

[1] https://pdfium-review.googlesource.com/97791

Bug: pdfium:1869
Change-Id: I43c6c9f16f3079549170528c86bfe1eee5b13833
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium_tests/+/100650
Reviewed-by: Lei Zhang <thestig@chromium.org>