Use the correct HTTP status code constant in NotModifiedException

NotModifiedException was using HttpServletResponse.SC_NOT_FOUND instead of SC_NOT_MODIFIED (likely an autocomplete typo).

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=182976671

java/google/registry/request/HttpException.java

@@ -84,7 +84,7 @@ public abstract class HttpException extends RuntimeException {
     }
 
     public NotModifiedException(String message) {
-      super(HttpServletResponse.SC_NOT_FOUND, message, null);
+      super(HttpServletResponse.SC_NOT_MODIFIED, message, null);
     }
 
     @Override