Update authenticating-to-the-rest-api-with-an-oauth-app.md

[Proman4713]

The primary check should be for refresh_token, not access_token, since that more closely resembles production environments where the user's access token will expire first. The rest of the code should play out the same, with the subsequent requests failing if the access token is absent or invalid

Why:

It initially misled me while I was trying to understand what GitHub's authentication flow with OAuth apps is supposed to be.

Closes: #45671

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

Before:

def authenticated?
  session[:access_token]
end

After:

def authenticated?
  session[:refresh_token]
end

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

[github-actions]

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Source	Review	Production	What Changed
apps/oauth-apps/building-oauth-apps/authenticating-to-the-rest-api-with-an-oauth-app.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

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