Fix missing "s" typo

Author: handrews

jsonschema-core.xml

@@ -198,7 +198,7 @@
                         <xref target="schema-document">schema document</xref> begins by applying
                         the <xref target="root">root schema</xref> to the complete instance
                         document.  From there, keywords known as applicators are used to determine
-                        which additional subschemas are applied.  Subschema may be applied in-place
+                        which additional subschemas are applied.  Subschemas may be applied in-place
                         to the current location, or to a child location.  The subschemas may be all
                         or part of the keyword's value, or the keyword's value may identify one
                         or more schemas in a way defined by the keyword.