Help/About: Fix typo and remove trailing spaces from some strings.

Props mukesh27, casiepa, davidbaumwald.
Merges [48809] to the 5.5 branch.
Fixes #51028.
Built from https://develop.svn.wordpress.org/branches/5.5@48811


git-svn-id: https://core.svn.wordpress.org/branches/5.5@48573 1a063a9b-81f0-0310-95a4-ce76da25c4cd