add free space map link in pg_freespacemap page

Hi,
I found that there is a link to `pg_freespacemap` in the `free space map`
page but there is no link to `free space map` in the `pg_freespacemap`
page.

diff --git a/doc/src/sgml/pgfreespacemap.sgml
b/doc/src/sgml/pgfreespacemap.sgml
index 5025498249..0ab3307e9c 100644
--- a/doc/src/sgml/pgfreespacemap.sgml
+++ b/doc/src/sgml/pgfreespacemap.sgml
@@ -9,7 +9,7 @@

<para>
The <filename>pg_freespacemap</filename> module provides a means for
examining the
- free space map (FSM). It provides a function called
+ <link linkend="storage-fsm">free space map</link> (FSM). It provides a
function called
<function>pg_freespace</function>, or two overloaded functions, to be
precise. The functions show the value recorded in the free space map for
a given page, or for all pages in the relation.

so I propose a patch to fix it.

---
Regards,
Lee Dong Wook.