docs: use alias in WHERE clause of full text search example

The current doc query specified an alias in the FROM clause and used in
it the target list, but not in the WHERE clause.

Reported-by: axykon@gmail.com

Discussion: https://postgr.es/m/158316348159.30450.16075357948244298217@wrigleys.postgresql.org

Backpatch-through: 9.5

Author: bmomjian

doc/src/sgml/textsearch.sgml

@@ -200,7 +200,7 @@ WHERE mid = 12;
 
 SELECT m.title || ' ' || m.author || ' ' || m.abstract || ' ' || d.body AS document
 FROM messages m, docs d
-WHERE mid = did AND mid = 12;
+WHERE m.mid = d.did AND m.mid = 12;
 </programlisting>
    </para>