doc: Fix example with __next__() in PL/Python function

Per PEP 3114, iterator.next() has been renamed to iterator.__next__(),
and one example in the documentation still used next().  This caused the
example provided to fail the function creation since Python 2 is not
supported anymore since 19252e8.

Author: Erik Wienhold
Discussion: https://postgr.es/m/173209043143.2092749.13692266486972491694@wrigleys.postgresql.org
Backpatch-through: 15

Author: michaelpq

doc/src/sgml/plpython.sgml

@@ -553,7 +553,7 @@ $$ LANGUAGE plpython3u;
 
     <varlistentry>
      <term>Iterator (any object providing <symbol>__iter__</symbol> and
-      <symbol>next</symbol> methods)</term>
+      <symbol>__next__</symbol> methods)</term>
      <listitem>
       <para>
 <programlisting>
@@ -569,7 +569,7 @@ AS $$
     def __iter__ (self):
       return self
 
-    def next (self):
+    def __next__(self):
       self.ndx += 1
       if self.ndx == len(self.who):
         raise StopIteration