I18N: Fix typo in a string in `rest_validate_value_from_schema()`.

Props pedromendonca.
Reviewed by TimothyBlynJacobs, SergeyBiryukov.
Merges [48767] to the 5.5 branch.
Fixes #50879.
Built from https://develop.svn.wordpress.org/branches/5.5@48768


git-svn-id: https://core.svn.wordpress.org/branches/5.5@48530 1a063a9b-81f0-0310-95a4-ce76da25c4cd