doc: Fix example command for ALTER FOREIGN TABLE ... OPTIONS.

In the documentation, previously the example command for
ALTER FOREIGN TABLE ... OPTIONS incorrectly included both
the option name and value with the DROP operation.
The correct syntax for the DROP operation requires only
the name of the option to be specified. This commit fixes
the example by removing the option value from the DROP operation.

Back-patch to all supported versions.

Author: Mehmet Emin KARAKAS <emin100@gmail.com>
Reviewed-by: Fujii Masao
Discussion: https://postgr.es/m/CANQrdXAHzbcEYhjGoe5A42OmfvdQhHFJzyKj9gJvHuDKyOF5Ng@mail.gmail.com

doc/src/sgml/ref/alter_foreign_table.sgml

@@ -521,7 +521,7 @@ ALTER FOREIGN TABLE distributors ALTER COLUMN street SET NOT NULL;
   <para>
    To change options of a foreign table:
 <programlisting>
-ALTER FOREIGN TABLE myschema.distributors OPTIONS (ADD opt1 'value', SET opt2 'value2', DROP opt3 'value3');
+ALTER FOREIGN TABLE myschema.distributors OPTIONS (ADD opt1 'value', SET opt2 'value2', DROP opt3);
 </programlisting></para>
 
  </refsect1>