Update plugin registry URL

[dotgrid]

Platforms affected

• Documentation Readme

Motivation and Context

• Fixes a broken URL

Description

The currently documented URL gives a 404. The new URL appears to be: https://cordova.apache.org/plugins/. This commit updates the URL in the Readme.

Testing

• Verified new URL is active

Checklist

• [x] I've run the tests to see all new and existing tests pass
• [x] I added automated test coverage as appropriate for this change
• [x] Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• [x] If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• [x] I've updated the documentation if necessary