fixing typo in NotSchema exception message

Author: erosb

core/src/main/java/org/everit/json/schema/NotSchema.java

@@ -59,7 +59,7 @@ public void validate(final Object subject) {
     } catch (ValidationException e) {
       return;
     }
-    throw new ValidationException(this, "subject must not be valid agains schema " + mustNotMatch,
+    throw new ValidationException(this, "subject must not be valid against schema " + mustNotMatch,
         "not");
   }