Fix ALTER TABLE doc examples.

Patch by Yugo Nagata <nagata@sraoss.co.jp>.  Confirmed by Amit
Langote, who is the original author of the document part.

Author: tatsuo-ishii

doc/src/sgml/ref/alter_table.sgml

@@ -1398,7 +1398,7 @@ ALTER TABLE cities
   <para>
    Detach a partition from partitioned table:
 <programlisting>
-ALTER TABLE cities
+ALTER TABLE measurement
     DETACH PARTITION measurement_y2015m12;
 </programlisting></para>