config.md: s/should/must/ for fields which cannot be evaluated

Signed-off-by: Erik Hollensbe <[email protected]>

Author: Erik Hollensbe

config.md

@@ -212,7 +212,7 @@ Note: Any OPTIONAL field MAY also be set to null, which is equivalent to being a
        This field is used to mark if the history item created a filesystem diff.
        It is set to true if this history item doesn't correspond to an actual layer in the rootfs section (for example, Dockerfile's [ENV](https://docs.docker.com/engine/reference/builder/#/env) command results in no change to the filesystem).
 
-Any extra fields in the Image JSON struct are considered implementation specific and should be ignored by any implementations which are unable to interpret them.
+Any extra fields in the Image JSON struct are considered implementation specific and MUST be ignored by any implementations which are unable to interpret them.
 
 Whitespace is OPTIONAL and implementations MAY have compact JSON with no whitespace.