docs: correct wording in rest-api/authentication docs

[Rawal27]

Closes: #ISSUE_NUMBER

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

Correct three occurrences of "There a few different ways to get a token" to "There are a few different ways to get a token" in the REST API authentication documentation.

No technical or behavioral changes are introduced.

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Source	Review	Production	What Changed
rest/authentication/authenticating-to-the-rest-api.md	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18 3.17	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18 3.17
rest/using-the-rest-api/getting-started-with-the-rest-api.md	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18 3.17	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18 3.17

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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[subatoi]

Thanks for spotting and fixing that, @Rawal27

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