Add missing "site."

reference/create-index-using-pgroonga.md

@@ -9,7 +9,7 @@ You need to specify `USING pgroonga` to `CREATE INDEX` to use PGroonga as index
 
 ## Syntax
 
-This section describes only `pgroonga` index method related `CREATE INDEX` syntax. See [CREATE INDEX document by PostgreSQL](http://www.postgresql.org/docs/{{ postgresql_short_version }}/static/sql-createindex.html) for full `CREATE INDEX` syntax.
+This section describes only `pgroonga` index method related `CREATE INDEX` syntax. See [CREATE INDEX document by PostgreSQL](http://www.postgresql.org/docs/{{ site.postgresql_short_version }}/static/sql-createindex.html) for full `CREATE INDEX` syntax.
 
 Here is syntax for creating single column index: