Fix typos

[ElizabethSamuel-MSFT]

Thank you for your pull request! Please provide the following information.

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Change Description:

Describe what the PR is about. 
Fixes a couple of typos in the content.

1. Do these changes impact any npm scripts commands (in package.json)? (e.g., running 'npm run start') No
   If Yes, briefly describe what is impacted.

2. Do these changes impact VS Code debugging options (launch.json)? No
   If Yes, briefly describe what is impacted.

3. Do these changes impact template output? (e.g., add/remove file, update file location, update file contents) No
   If Yes, briefly describe what is impacted.

4. Do these changes impact documentation? (e.g., a tutorial on https://docs.microsoft.com/en-us/office/dev/add-ins/overview/office-add-ins) Yes
   If Yes, briefly describe what is impacted.
   May need to update add-in command screenshots since the button text was changed

If you answered yes to any of these please do the following:
> Include 'Rick-Kirkham' in the review
> Make sure the README file is correct

Validation/testing performed:

Describe manual testing done.