improve description of version field in traceparent request header

This fixes the confusing wording in the description of the version
field, which previously didn't clearly distinguish between a possible
internal representation (1 byte) and the format on the wire
(two ascii chars). New wording (mostly) contributed by @aphillips.

fixes w3c#507

spec/20-http_request_header_format.md

@@ -83,9 +83,7 @@ The dash (`-`) character is used as a delimiter between fields.
 version = 2HEXDIGLC ; this document assumes version 00. Version ff is forbidden
 ```
 
-The value is US-ASCII encoded (which is UTF-8 compliant).
-
-Version (`version`) is 1 byte representing an 8-bit unsigned integer. Version `ff` is invalid. The current specification assumes the `version` is set to `00`.
+Version (`version`) is an 8-bit unsigned integer value, serialized as an ASCII string with two characters. Version 255 (`"ff"`) is invalid. This specification assumes the version is set to 0 (`"00"`).
 
 #### version-format