[Impeller] Fix a misspelling and name mismatch in a shader test fixture#50983

auto-submit[bot] merged 1 commit intoflutter:mainflutter/engine:mainfrom

jason-simmons:iplr_sepia_fixture_interp_spelljason-simmons/flutter_engine:iplr_sepia_fixture_interp_spellCopy head branch name to clipboard