Update creating-diagrams.md

[bgrzesiak-fetch]

Fix a couple typos

Why:

Closes: #34480

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

Just some grammar.

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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/working-with-advanced-formatting/creating-diagrams.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]

@bgrzesiak-fetch Thanks so much for opening a PR with this fix! I'll get this merged 🍏

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