Update haddock on assertEq to specify the expected and actual value ordering

[jkeuhlen]

Before submitting your PR, check that you've:

• Bumped the version number
  (Documentation updates don't need a version number bump)
• Documented new APIs with Haddock markup
  (Just improving documentation to match the docs for the underlying HUnit implementation.)
• Added @since declarations to the Haddocks for new, public APIs
  N/A

After submitting your PR:

• Update the Changelog.md file with a link to your PR
• Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)

[jkeuhlen]

I'm unsure if this warrants a changelog update or not, but the impetus here is probably ~half of the tests in our codebase have this ordering backwards since most people just look at the yesod docs instead of the HUnit docs, so this makes them line up better!

[jezen]

Better documentation is always welcome. Thank you 👍