Fix the unclear SHOULD requirement for "gzip" OTLP content type (#1038)

MUST is the correct requirement here. There is no situation when a different content type is a better choice.

specification/protocol/otlp.md

@@ -323,7 +323,7 @@ The client MUST set "Content-Type: application/x-protobuf" request header when
 sending binary-encoded Protobuf or "Content-Type: application/json" request
 header when sending JSON encoded Protobuf payload.
 
-The client MAY gzip the content and in that case SHOULD include
+The client MAY gzip the content and in that case MUST include
 "Content-Encoding: gzip" request header. The client MAY include
 "Accept-Encoding: gzip" request header if it can receive gzip-encoded responses.