Correct documentation about LayoutTest's requirement for "-expected.txt" files

[ghost]

https://docs.webkit.org/Build%20%26%20Debug/Tests.html#correctness-testing-in-webkit

mentions

There are a few styles of layout tests but all of them have a test file and expected result (ends with -expected.txt), and the test passes if the test file’s output matches that of the expected result.

Some styles of layout tests, e.g. web-platform reftests ^1, seem to require a "-expected.html" file instead. See e.g. https://searchfox.org/wubkat/search?q=&path=font-variant-ligatures-01&case=true&regexp=false.