Fix duplicated word in default exception template

[xcompass]

Summary

Fixes a duplicated word in templates/exception_default.html.ep. The paragraph asking users to report errors to the site's webmaster currently reads:

... including all of the following information as well as what what you were doing when the error occurred.

This removes the duplicated "what" so the sentence reads correctly. No functional or markup changes — a one-word text fix in the default exception page shown to end users when an unhandled error occurs.

Testing

• Verified the duplicated word is present in the current WeBWorK-2.21 branch (and develop) and that the diff is limited to removing the single extra "what" on that line (one line changed, nothing else).
• Confirmed the file is a Mojolicious .html.ep template, which is not covered by the repository's perltidy (*.pl/*.pm/*.t/*.at) or prettier (htdocs/**/*.{js,css,scss,html}) format checks, so no formatting tooling applies.

Dependencies: none

[xcompass]

Sending a few PRs from our local improvements. Happy to switch the target branch to develop if needed.

[drgrice1]

Looks good. Could you please shorten what you say when you make these pull requests? Is it generated by AI? There is a lot of wording when all that is needed would have been "Fix a typo".

[xcompass]

Will do.