Docs: adjust pg_upgrade syntax to mark -B as optional

This was made optional in 959f6d6a.

Author: Justin Pryzby
Discussion: https://postgr.es/m/20220411020336.GB26620@telsasoft.com
Backpatch-through: 13, where -B was made optional

doc/src/sgml/ref/pgupgrade.sgml

@@ -24,8 +24,7 @@ PostgreSQL documentation
    <command>pg_upgrade</command>
    <arg choice="plain"><option>-b</option></arg>
    <arg choice="plain"><replaceable>oldbindir</replaceable></arg>
-   <arg choice="plain"><option>-B</option></arg>
-   <arg choice="plain"><replaceable>newbindir</replaceable></arg>
+   <arg choice="opt"><option>-B</option> <replaceable>newbindir</replaceable></arg>
    <arg choice="plain"><option>-d</option></arg>
    <arg choice="plain"><replaceable>oldconfigdir</replaceable></arg>
    <arg choice="plain"><option>-D</option></arg>