Doc: Python's control flow construct is try/except not try/catch.

Very ancient thinko, dating evidently to 22690719e.
Spotted by gweatherby.

Discussion: https://postgr.es/m/170423637139.1288848.11840082988774620003@wrigleys.postgresql.org

doc/src/sgml/plpython.sgml

@@ -1175,7 +1175,7 @@ plan = plpy.prepare("INSERT INTO operations (result) VALUES ($1)", ["text"])
 plpy.execute(plan, [result])
 $$ LANGUAGE plpython3u;
 </programlisting>
-    Note that the use of <literal>try/catch</literal> is still
+    Note that the use of <literal>try</literal>/<literal>except</literal> is still
     required.  Otherwise the exception would propagate to the top of
     the Python stack and would cause the whole function to abort with
     a <productname>PostgreSQL</productname> error, so that the