Help/About: String update for 5.5 release.

We don't use hyphens for the [block editor](https://make.wordpress.org/core/handbook/best-practices/spelling/).

See #50416.
Props davidbaumwald, marybaum, desrosj.

Built from https://develop.svn.wordpress.org/branches/5.5@48723


git-svn-id: https://core.svn.wordpress.org/branches/5.5@48485 1a063a9b-81f0-0310-95a4-ce76da25c4cd