removes broken link in README

[benironside]

Very minor fix removing a broken link in the repo's README.

[github-actions]

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[mergify]

This pull request does not have a backport label. Could you fix it @benironside? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• v7.x is the label to automatically backport to the 7.x branch.
• v7./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[joepeeples]

While we're fixing this link, how about also changing the link in step 2 below ("Open an issue using the appropriate template") to point to the issue selection page here, instead of the file browser view: https://github.com/elastic/security-docs/issues/new/choose