docs(fab): document correct ion-icon usage for vue

[liamdebeasi]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Build (npm run build) was run locally and any changes were pushed
• Lint (npm run lint) has passed locally and any fixes were made for failures

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

See: https://forum.ionicframework.com/t/ion-icon-not-visible-on-ion-fab-button/209961/3
Incorrect ion-icon usage was documented for vue, so icons were not showing up.

What is the new behavior?

• Switched example to use local icon registration in the fab docs

Does this introduce a breaking change?

• Yes
• No

Other information