doc: Add random_zipfian to list of random functions with argument.

Author: Ildar Musin
Reviewed-By: Fabian Coelho
Discussion: https://postgr.es/m/6376ed81-3ce8-14f4-4758-099872f4ce7d@postgrespro.ru

doc/src/sgml/ref/pgbench.sgml

@@ -1332,9 +1332,10 @@ pgbench <optional> <replaceable>options</replaceable> </optional> <replaceable>d
    <para>
     The <literal>random</literal> function generates values using a uniform
     distribution, that is all the values are drawn within the specified
-    range with equal probability. The <literal>random_exponential</literal> and
-    <literal>random_gaussian</literal> functions require an additional double
-    parameter which determines the precise shape of the distribution.
+    range with equal probability. The <literal>random_exponential</literal>, 
+    <literal>random_gaussian</literal> and <literal>random_zipfian</literal>
+    functions require an additional double parameter which determines the precise
+    shape of the distribution.
    </para>
 
    <itemizedlist>