Change to MUST

Author: ianswett

draft-ietf-moq-transport.md

@@ -1233,7 +1233,7 @@ AbsoluteRange (0x4):  Specifies a closed subscription starting at StartObject
 in StartGroup and ending at EndObject in EndGroup.  The start and end of the
 range are inclusive.  EndGroup and EndObject MUST specify the same or a later
 object than StartGroup and StartObject. If the StartGroup is prior
-to the current group, the publisher SHOULD reply with a
+to the current group, the publisher MUST reply with a
 SUBSCRIBE_ERROR with code 'Invalid Range'.
 
 A filter type other than the above MUST be treated as error.