Add id attribute to reftitle to allow cross referencing from the new

 vaccumdb reference page.

Author: Thomas G. Lockhart

doc/src/sgml/ref/vacuum.sgml

@@ -1,6 +1,6 @@
 <REFENTRY ID="SQL-VACUUM-1">
 <REFMETA>
-<REFENTRYTITLE>
+<REFENTRYTITLE id="vacuum">
 VACUUM
 </REFENTRYTITLE>
 <REFMISCINFO>SQL - Language Statements</REFMISCINFO>