doc: Fix table cell overflow

Fix one instance of a table cell overflow by adding a zero-width
space.  The visual impact of this is minimal, but since this is
currently the only such case reported by FOP ("contents of ... exceed
the available area"), it seems worth getting rid of.

doc/src/sgml/high-availability.sgml

@@ -305,7 +305,7 @@ protocol to make nodes agree on a serializable transactional order.
      <entry>File System Repl.</entry>
      <entry>Write-Ahead Log Shipping</entry>
      <entry>Logical Repl.</entry>
-     <entry>Trigger-Based Repl.</entry>
+     <entry>Trigger-&zwsp;Based Repl.</entry>
      <entry>SQL Repl. Middle-ware</entry>
      <entry>Async. MM Repl.</entry>
      <entry>Sync. MM Repl.</entry>