USWDS-Site- accordion: Update multiselectable docs

[amyleadem]

Summary

Update accordion documentation to use the correct multi-select attribute.

Our current accordion documentation tells users to add the aria-multiselectable="true" attribute to trigger a multi-select accordion group, but the attribute is actually data-allow-multiple.

Correcting this will make sure that users are able to successfully trigger the multi-select variant, while also preventing accessibility errors.

Related issue

Closes uswds/uswds#4865

Preview link

Preview link: Accordion component page

To confirm:

1. Open the accordion twig file in USWDS
2. Confirm that our templates use data-allow-multiple instead of aria-multiselectable="true" (In fact, there is no reference to aria-multiselectable in the USWDS code base.)

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Before opening this PR, make sure you’ve done whichever of these applies to you:

• Confirm that this code follows the 18F Front End Coding Style Guide and Accessibility Guide.
• Run git pull origin [base branch] to pull in the most recent updates from your base and check for merge conflicts. (Often, the base branch is develop).
• Run npm test and confirm that all tests pass.
• Run your code through HTML_CodeSniffer and make sure it’s error free.

[mejiaj]

LGTM, thanks!