Remove misleading "must" in ref.name requirements

Signed-off-by: Tianon Gravi <[email protected]>

annotations.md

@@ -33,7 +33,7 @@ This specification defines the following annotation keys, intended for but not l
 - **org.opencontainers.image.ref.name** Name of the reference for a target (string).
   - SHOULD only be considered valid when on descriptors on `index.json` within [image layout](image-layout.md).
   - Character set of the value SHOULD conform to alphanum of `A-Za-z0-9` and separator set of `-._:@/+`
-  - The reference must match the following [grammar](considerations.md#ebnf):
+  - A valid reference matches the following [grammar](considerations.md#ebnf):
 
     ```ebnf
     ref       ::= component ("/" component)*