Fix typo

Author: Relequestual

jsonschema-core.xml

@@ -1328,7 +1328,7 @@
                             as "original" as it is the schema to be extended, describes
                             an object with one string property and one recursive reference
                             property, "r".  The second schema, identified as "extension",
-                            references the first, and describes an additional things" property,
+                            references the first, and describes an additional "things" property,
                             which is an array of recursive references.
                             It also repeats the description of "r" from the original schema.
                         </preamble>