Update oauth.md

[smarvin]

W-18443839: adding requested note for clarity

(I don't have access to the settings, so I can't see if you're publishing the docs from gh-pages. I'm assuming so. Let me know if I should be using a different branch.)

Things To Remember:

1. Be sure to merge into the right branch. The choices are:

• master if your PR runs on the current version of Tableau.
• dev-2021.2

2. You should update at least one of the following:

• The changelog of the project you modified.
• The root README.md if the PR is a release of the SDK, Connector Packager, or TDVT.
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