Doc: fix syntax description for psql's \setenv.

The variable name isn't optional --- looks like a copy-and-paste-o from
the \set command, where it is.

Dilip Kumar

Author: tglsfdc

doc/src/sgml/ref/psql-ref.sgml

@@ -2399,7 +2399,7 @@ lo_import 152801
 
 
       <varlistentry>
-        <term><literal>\setenv [ <replaceable class="parameter">name</replaceable> [ <replaceable class="parameter">value</replaceable> ] ]</literal></term>
+        <term><literal>\setenv <replaceable class="parameter">name</replaceable> [ <replaceable class="parameter">value</replaceable> ]</literal></term>
 
         <listitem>
         <para>