Fix #35823 Use backticks to format references to the file name

[adi-075]

Use backticks to format references to the file names and Remove Italics from placeholder text

Why:

According to the style guide, you should use backticks to format references to file names and directory names in a monospaced font. Also, removed italics from placeholder text.

Closes: #35823
Closes: #35822

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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Content directory changes

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Source	Preview	Production	What Changed
pages/setting-up-a-github-pages-site-with-jekyll/adding-a-theme-to-your-github-pages-site-using-jekyll.md	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10

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

[nguyenalex836]

@adi-075 Thanks so much for opening a PR with this fix! I'll update the branch and get this merged once tests are passing 🍏

[github-actions]

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[adi-075]

Thank you for merging this PR @nguyenalex836!