Update envelope.md

Co-authored-by: Mark Lodato <[email protected]>

Author: AdamZWu

envelope.md

@@ -69,7 +69,7 @@ envelopes with individual signatures.
 ### Parsing rules
 
 *   The following fields are REQUIRED and MUST be set, even if empty:
-    one of (`payload`, `payloadUtf8`), `payloadType`, `signature`, `signature.sig`.
+    exactly one of {`payload` or `payloadUtf8`}, `payloadType`, `signature`, `signature.sig`.
 *   The following fields are OPTIONAL and MAY be unset: `signature.keyid`.
     An unset field MUST be treated the same as set-but-empty.
 *   Producers, or future versions of the spec, MAY add additional fields.