Missed a "schema document" -> "schema resource"

This was just an oversight regarding where $vocabualry is legal.
It should have always been "schema resource root."

Author: handrews

jsonschema-core.xml

@@ -1273,7 +1273,7 @@
                     </t>
                     <t>
                         The "$vocabulary" keyword SHOULD be used in the root schema of any schema
-                        document intended for use as a meta-schema.  It MUST NOT appear in subschemas.
+                        resource intended for use as a meta-schema.  It MUST NOT appear in subschemas.
                     </t>
                     <t>
                         The "$vocabulary" keyword MUST be ignored in schema documents that