use backticks to format _layouts directory in securing-your-github-pages-site-with-https#resolving-problems-with-mixed-content

[bocsir]

Why:

Closes: #35794

What's being changed (if available, include any code snippets, screenshots, or gifs):

Added back tics around _layouts directory in file specified in title.

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require a SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing.

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[github-actions]

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The table contains an overview of files in the content directory that have been changed in this pull request. It's provided to make it easy to review your changes on the staging site. Please note that changes to the data directory will not show up in this table.

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Content directory changes

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Source	Preview	Production	What Changed
pages/getting-started-with-github-pages/securing-your-github-pages-site-with-https.md	fpt ghec	fpt ghec

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fpt: Free, Pro, Team
ghec: GitHub Enterprise Cloud
ghes: GitHub Enterprise Server

[nguyenalex836]

@bocsir Thank you so much for opening a PR with this fix! ✨ We are currently in a repo freeze, but we'll get this merged as soon as the freeze ends on Jan 6th 💛