update other MUST to SHOULD

Author: gregsdennis

jsonschema-core.md

@@ -1144,7 +1144,7 @@ provides the ability to validate recursive structures through self-reference.
 The resolved IRI produced by these keywords is not necessarily a network
 locator, only an identifier. A schema need not be downloadable from the address
 if it is a network-addressable URL. Implementations which can access the network
-MUST default to operating offline.
+SHOULD default to operating offline.
 
 ##### Direct References with `$ref` {#ref}