use 'required'

Author: leebyron

spec/Appendix B -- Notation Conventions.md

@@ -249,7 +249,7 @@ ordered map, which does not have an entry with that key, is ordered after
 existing entries.
 
 Note: This specification defines ordered data collection only when strictly
-necessary. When an order is observable, implementations should preserve it to
+required. When an order is observable, implementations should preserve it to
 improve output legibility and stability. For example if applying a grammar to an
 input string yields a _set_ of elements, serialization should emit those
 elements in the same source order.