doc: Fix logical replication protocol doc detail

Author: Masahiko Sawada <sawada.mshk@gmail.com>
Reported-by: Kyle Conroy <kyle@kyleconroy.com>
Bug: #14775

doc/src/sgml/protocol.sgml

@@ -5990,6 +5990,16 @@ Commit
 </listitem>
 </varlistentry>
 <varlistentry>
+<term>
+        Int8
+</term>
+<listitem>
+<para>
+                Flags; currently unused (must be 0).
+</para>
+</listitem>
+</varlistentry>
+<varlistentry>
 <term>
         Int64
 </term>