Fix formatting for Microsoft Office documents list

[ahmedazhar05]

Why:

Currently the documentation for Work with advanced formatting / Attaching files which states the list of Microsoft Office documents files that can be attached to the issues and pull requests has a typo that was introduced 10 months ago in 2de13b5 commit and hence required non-critical minor fixing to look consistent.

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

Change from:

• .xls`` ,.xlsm

Change to:

• .xls ,.xlsm

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 an 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 and the changes look good in the review environment.

[github-actions]

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Source	Review	Production	What Changed
get-started/writing-on-github/working-with-advanced-formatting/attaching-files.md	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17

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

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