oggopus: Revert SHOULD->MUST for reserved mappings.

Mark Harris convinced me that the significant delay between "WG
 consensus" and "RFC" means we shouldn't rely on RFC updates to
 give people permission to start deploying new things.

doc/draft-ietf-codec-oggopus.xml

@@ -925,8 +925,8 @@ Implementations SHOULD NOT produce output for channels mapped to stream index
  title="Undefined Channel Mappings">
 <t>
 The remaining channel mapping families (2...254) are reserved.
-An implementation encountering a reserved channel mapping family value MUST act
- as though the value is 255.
+A demuxer implementation encountering a reserved channel mapping family value
+ SHOULD act as though the value is 255.
 </t>
 </section>