docs: minor improvement to aria-describedby doc

[huyenltnguyen]

Description

This PR adds some minor improvement to https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Attributes/aria-describedby.

Motivation

I find some sentences in the doc a little hard to understand, so I'm proposing a couple changes to hopefully improve them.

Additional details

N/A

Related issues and pull requests

N/A

[github-actions[bot]]

Preview URLs

• /en-US/docs/Web/Accessibility/ARIA/Attributes/aria-describedby

[bsmth]

Thanks for opening this one. @scottaohara do you want to have a look or should I ask someone from the team to review? Thanks!