REST API: Merge similiar error message strings in the Terms Controller.

Replaces the "doesn't" contraction with "does not" to be consistent with similar strings in `WP_Error` messages when a specified term or parent term is missing in `WP_REST_Terms_Controller`.

Props ramiy, ocean90.
Fixes #39176.

Built from https://develop.svn.wordpress.org/trunk@39648


git-svn-id: https://core.svn.wordpress.org/trunk@39588 1a063a9b-81f0-0310-95a4-ce76da25c4cd