clarify "against"

Co-authored-by: Karen Etheridge <[email protected]>

Author: gregsdennis

jsonschema-core.md

@@ -917,7 +917,7 @@ resources, unless such a resource itself declares a different dialect by
 including the `$schema` keyword with a different value.
 
 The value of this keyword MUST be an [IRI](#rfc3987) (containing a scheme) and
-this IRI MUST be normalized. The current schema MUST be valid against the
+this IRI MUST be normalized. The current schema MUST be valid when evaluated against the
 meta-schema identified by this IRI.
 
 If this IRI identifies a retrievable resource, that resource SHOULD be of media