docs: Fix column count attribute in table

Nothing seems to actually depend on the attribute, as the docs built
successfully, but let's be tidy.

Reported offlist by Matthias van de Meent

Author: hlinnaka

doc/src/sgml/protocol.sgml

@@ -223,7 +223,7 @@
    <table>
     <title>Protocol versions</title>
 
-    <tgroup cols="2">
+    <tgroup cols="3">
      <thead>
       <row>
        <entry>Version</entry>