schema/taxonomy_term: Updated description

Updated description of `taxonomy_term.language` to include support for
ISO 639-3 as per proposal:

* https://docs.google.com/document/d/1pXX7hQbZ5w8FCJrBjlc3U_jZcdS4URQJrSU1EbnF2bc/view

schema/taxonomy_term.json

@@ -86,7 +86,7 @@
             "name": "language",
             "type": "string",
             "title": "Language",
-            "description": "An ISO 639-1, or ISO 639-2 [language code](available at http://www.loc.gov/standards/iso639-2/php/code_list.php) to represent the language of the term. The three-letter codes from ISO 639-2 provide greater accuracy when describing variants of languages, which may be relevant to particular communities.",
+            "description": "An ISO 639-1, ISO 639-2, or ISO 639-3 [language code](available at http://www.loc.gov/standards/iso639-2/php/code_list.php) to represent the language of the term. The three-letter codes from ISO 639-2 and ISO 639-3 provide greater accuracy when describing variants of languages, which may be relevant to particular communities.",
             "constraints": {
                 "unique": false
             },
@@ -129,4 +129,4 @@
         "name",
         "description"
     ]
-}
+}