Fix documentation for pgbench tpcb-like.

We choose a random value for delta, not balance.  Back-patch to 9.6 where
the mistake arrived.

Author: Fabien Coelho
Discussion: https://postgr.es/m/alpine.DEB.2.21.1904081752210.5867@lancre

doc/src/sgml/ref/pgbench.sgml

@@ -710,7 +710,7 @@ pgbench <optional> <replaceable>options</> </optional> <replaceable>dbname</>
   <para>
    The default built-in transaction script (also invoked with <option>-b tpcb-like</>)
    issues seven commands per transaction over randomly chosen <literal>aid</>,
-   <literal>tid</>, <literal>bid</> and <literal>balance</>.
+   <literal>tid</>, <literal>bid</> and <literal>delta</>.
    The scenario is inspired by the TPC-B benchmark, but is not actually TPC-B,
    hence the name.
   </para>