Fix comment about name (#801)

Fixes #720 

The `name` production matches NCName except that we exclude U+FFFD. This updates the comment in the ABNF.

Author: aphillips

spec/message.abnf

@@ -72,7 +72,7 @@ reserved-body-part        = reserved-char / escaped-char / quoted
 
 ; Names and identifiers
 ; identifier matches https://www.w3.org/TR/REC-xml-names/#NT-QName
-; name matches https://www.w3.org/TR/REC-xml-names/#NT-NCName
+; name matches https://www.w3.org/TR/REC-xml-names/#NT-NCName but excludes U+FFFD
 identifier = [namespace ":"] name
 namespace  = name
 name       = name-start *name-char