Merge pull request #1037 from ssilverman/add-a-word

[1036] Add a missing "a"

Author: Relequestual

jsonschema-core.xml

@@ -1844,7 +1844,7 @@
 
             <section title="Compound Documents">
                 <t>
-                    A Compound Schema Document is defined as JSON document (sometimes called a "bundled" schema)
+                    A Compound Schema Document is defined as a JSON document (sometimes called a "bundled" schema)
                     which has multiple embedded JSON Schema Resources bundled into the same document to
                     ease transportation.
                 </t>