Fix typo in protocol.sgml.

The 'Stream Stop' message is misspelled as 'Stream End' in the docs.

Author: Masahiko Sawada
Backpatch-through: 14, where it was introduced
Discussion: https://postgr.es/m/CAD21AoDeScrsHhLyEPYqN3sydg6PxAPVBboK=30xJfUVihNZDA@mail.gmail.com

doc/src/sgml/protocol.sgml

@@ -7178,7 +7178,7 @@ Truncate
 
 <para>
 
-The following messages (Stream Start, Stream End, Stream Commit, and
+The following messages (Stream Start, Stream Stop, Stream Commit, and
 Stream Abort) are available since protocol version 2.
 
 </para>