Docs: Correct some inconsistencies in comments with Dashicon class references...

Docs: Correct some inconsistencies in comments with Dashicon class references in `wp-admin/css/customize-widgets.css`.

Prepend all class name references with `dashicons-` for clarity.

Props 1naveengiri, marcio-zebedeu, hareesh-pillai, SergeyBiryukov.
Fixes #48417.
Built from https://develop.svn.wordpress.org/trunk@46681


git-svn-id: https://core.svn.wordpress.org/trunk@46481 1a063a9b-81f0-0310-95a4-ce76da25c4cd