Updating verbiage on UXP API reference head page

[chobberoni]

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Topic

This is a pull request for:

• [x] A tutorial contained within this repo.
• [ ] A sample contained within this repo.
• [ ] Something new that I have already discussed with Adobe in a GitHub issue. Issue link:

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Description of the pull request

Changed "three categories" to "these primary categories" as there are now 5 core categories.