s/SHALL/MUST/ (for clarity)

Author: awwright

jsonschema-core.xml

@@ -298,7 +298,7 @@
                 Any time a subschema is expected, a schema may instead use an object containing a "$ref" property.
                 The value of the $ref is a URI Reference.
                 Resolved against the current URI base, it identifies the URI of a schema to use.
-                All other properties in a "$ref" object SHALL be ignored.
+                All other properties in a "$ref" object MUST be ignored.
             </t>
             <t>
                 The URI is not a network locator, only an identifier. A schema need not be downloadable from the address