Editorial: obsolete RFC reference, terminology, awkward phrasing

[clemensv]

The following minor editorial issues were found during review:

1. Obsolete RFC4646 reference

RFC4646 in the references block is obsolete. It should be updated to RFC5646 (the current BCP 47 reference for language tags).

2. Terminology in `` description

The text says:

adding the JSONSchemaValidation key to the `` clause

is an array (not a "clause"), and `JSONSchemaValidation` is an entry (not a "key"). Suggested fix: "adding `JSONSchemaValidation` to the array".

3. Awkward parenthetical in has keyword definition

The has keyword validates that an object or map has at least one (property) value that matches the schema.

The parenthetical "(property)" is unnecessary and reads awkwardly. Suggest removing it.