Fix START_REPLICATION syntax in document.

Enclose "TIMELINE tli" part with brackets because it's optional.

Backport to 9.3 where TIMELINE option was introduced.

Noted by Marko Tiikkaja

Author: MasaoFujii

doc/src/sgml/protocol.sgml

@@ -1400,12 +1400,14 @@ The commands accepted in walsender mode are:
   </varlistentry>
 
   <varlistentry>
-    <term>START_REPLICATION <replaceable class="parameter">XXX/XXX</> TIMELINE <replaceable class="parameter">tli</></term>
+    <term>START_REPLICATION <replaceable class="parameter">XXX/XXX</>  [<literal>TIMELINE</literal> <replaceable class="parameter">tli</>]</term>
     <listitem>
      <para>
       Instructs server to start streaming WAL, starting at
-      WAL position <replaceable class="parameter">XXX/XXX</> on timeline
-      <replaceable class="parameter">tli</>.
+      WAL position <replaceable class="parameter">XXX/XXX</>.
+      If <literal>TIMELINE</literal> option is specified,
+      streaming starts on timeline <replaceable class="parameter">tli</>;
+      otherwise, the server's current timeline is selected.
       The server can reply with an error, e.g. if the requested section of WAL
       has already been recycled. On success, server responds with a
       CopyBothResponse message, and then starts to stream WAL to the frontend.