Add mention of composite actions under "Types of actions" in about-custom-actions.md

[mhucka]

Code of Conduct

• [X] I have read and agree to the GitHub Docs project's Code of Conduct

What article on docs.github.com is affected?

content/actions/creating-actions/about-custom-actions.md

What part(s) of the article would you like to see updated?

Change the first sentence of the paragraph under the heading "Types of Actions" to mention composite actions.

Additional information

The first sentence of the paragraph under the heading "Types of Actions" only mentioned Docker and JavaScript actions, but the table right below that paragraph contains 3 types of actions. The first sentence of the paragraph could be modified to mention composite actions to make it more complete.

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[cmwilson21]

@mhucka Thanks for opening an issue and submitting a PR! ✨