math/rand: fix example_test to show with the correct method

Originally it used r.Int63() to show "Uint32", and now we use the correct r.Uint32() method.

Fixes #9429

Change-Id: I8a1228f1ca1af93b0e3104676fc99000257c456f
Reviewed-on: https://go-review.googlesource.com/2069
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>