doc: Fix column_name parameter in ALTER MATERIALIZED VIEW

Parameter column_name must be an existing column because ALTER
MATERIALIZED VIEW cannot add new columns.  The old description was
likely copied from ALTER TABLE.

Author: Erik Wienhold
Discussion: https://postgr.es/m/6880ca53-7961-4eeb-86d5-6bd05fc2027e@ewie.name
Backpatch-through: 12

doc/src/sgml/ref/alter_materialized_view.sgml

@@ -96,7 +96,7 @@ ALTER MATERIALIZED VIEW ALL IN TABLESPACE <replaceable class="parameter">name</r
      <term><replaceable class="parameter">column_name</replaceable></term>
      <listitem>
       <para>
-       Name of a new or existing column.
+       Name of an existing column.
       </para>
      </listitem>
     </varlistentry>