remove unused manifest property and fix spelling

[Rick-Kirkham]

Change Description:

1. The "displayName" property is only used in add-ins that have custom keyboard shortcuts. None of our templates have this feature.

2. "task pane" should be 2 words

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

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

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

6. Do these changes impact documentation? (e.g., a tutorial on https://docs.microsoft.com/en-us/office/dev/add-ins/overview/office-add-ins)
   If Yes, briefly describe what is impacted.
   No

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.