Fix duplication in ALTER MATERIALIZE VIEW synopsis

Commit 3c4cf08 should have removed SET TABLESPACE from the synopsis
of ALTER MATERIALIZE VIEW as a possible "action" when it added a
separate line for it in the main command listing, but failed to.
Repair.

Backpatch to 9.4, like the aforementioned commit.

Author: alvherre

doc/src/sgml/ref/alter_materialized_view.sgml

@@ -45,7 +45,6 @@ ALTER MATERIALIZED VIEW ALL IN TABLESPACE <replaceable class="parameter">name</r
     SET ( <replaceable class="PARAMETER">storage_parameter</replaceable> = <replaceable class="PARAMETER">value</replaceable> [, ... ] )
     RESET ( <replaceable class="PARAMETER">storage_parameter</replaceable> [, ... ] )
     OWNER TO { <replaceable class="PARAMETER">new_owner</replaceable> | CURRENT_USER | SESSION_USER }
-    SET TABLESPACE <replaceable class="PARAMETER">new_tablespace</replaceable>
 </synopsis>
  </refsynopsisdiv>