Fix typo

Author: ucarion

jsonschema-core.xml

@@ -40,7 +40,7 @@
                 <email>[email protected]</email>
             </address>
         </author>
-        
+
         <author fullname="Ben Hutton" initials="B" surname="Hutton" role="editor">
             <organization>Wellcome Sanger Institute</organization>
             <address>
@@ -156,7 +156,7 @@
                 </t>
                 <t>
                     Evaluation of a parent schema object can complete once all of its
-                    subschemas have been evaluated, although in some circumstance evaluation
+                    subschemas have been evaluated, although in some circumstances evaluation
                     may be short-circuited due to assertion results.
                 </t>
                 <section title="Keyword Interactions">