docs: change to correct anchor for reporting errors

[fiskhandlarn]

Goal

the current url leads to a non-existant anchor on https://docs.bugsnag.com/platforms/javascript/

Changeset

see commit

Testing

i clicked the new link whilst previewing the changes and https://docs.bugsnag.com/platforms/javascript/ scrolled to Reporting handled errors

[mclack]

Hi @fiskhandlarn

Thanks for raising this. The change looks good.

External users can't merge PRs directly on our repos, so I've raised another PR for this which includes your commit to ensure you are credited for the contribution: #2563