Fix microscopic typo in Core spec

Author: tiangolo

jsonschema-core.xml

@@ -1916,7 +1916,7 @@ User-Agent: product-name/5.4.1 so-cool-json-schema/1.0.2 curl/7.43.0
             </t>
             <t>
                 Servers MUST ensure that malicious parties can't change the functionality of
-                existing schemas by uploading a schema with an pre-existing or very similar "$id".
+                existing schemas by uploading a schema with a pre-existing or very similar "$id".
             </t>
             <t>
                 Individual JSON Schema vocabularies are liable to also have their own security