Fix operator Python example for -builtin

Output was incorrect for -builtin as custom __str__ was not being
called.

Examples/python/operator/example.i

@@ -17,6 +17,7 @@
 %include "example.h"
 
 /* An output method that turns a complex into a short string */
+%feature("python:slot", "tp_str", functype="reprfunc") Complex::__str__; // For -builtin option to use __str__ in the tp_str slot
 %extend Complex {
    char *__str__() {
        static char temp[512];