Update quickstart-for-writing-on-github.md

[word-wiz]

Fixes incorrect spelling 'expandible'.

Why:

Closes: #34552

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

Fixes spelling

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Content directory changes

You may find it useful to copy this table into the pull request summary. There you can edit it to share links to important articles or changes and to give a high-level overview of how the changes in your pull request support the overall goals of the pull request.

Source	Preview	Production	What Changed
get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/quickstart-for-writing-on-github.md	fpt ghec ghes@ 3.14 3.13 3.12 3.11 3.10	fpt ghec ghes@ 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]

@word-wiz Thank you so much for opening a PR with this fix! ✨ This repo is currently in a code freeze, but we will get this merged as soon as the freeze ends on 9/16 🍏

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