Fix schema->subschema in prefixItems description

Author: handrews

jsonschema-core.xml

@@ -2349,7 +2349,7 @@
                         </t>
                         <t>
                             Validation succeeds if each element of the instance validates
-                            against the schema at the same position, if any.  This keyword
+                            against the subschema at the same position, if any.  This keyword
                             does not constrain the length of the array.  Only array positions
                             present in both the keyword's value and the instance value are
                             affected by this keyword.