Fix anchor in pre-receive secret detection documentation link

What does this MR do and why?

This merge request fixes the anchor in the documentation link that is displayed when pre-receive secret detection blocks a git push, as the link gets html-sanitized due to the use of help_page_url helper method, and this result in %23 being displayed instead of the # in the link anchor. Please see the screenshots below for an example of before and after.

MR acceptance checklist

I have evaluated this MR against the MR acceptance checklist.

Screenshots

Before	After