Fix the typo in a corpus test filename.

Rename "formfeild.pdf" to "formfield.pdf" and rename its expectation.

Change-Id: Id38155b396aa652cb26e1638698ebc4d7260ae69
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium_tests/+/104931
Reviewed-by: Tom Sepez <tsepez@chromium.org>