math/rand: example_test should show examples with the correct Uint32 method

[guobiao]

In math/rand's example_test.go, line 75: http://golang.org/src/math/rand/example_test.go#L75.

I believe the intention was to show with r.Uint32() rather than the written r.Int63(). Fortunately, the underlying implementation for both methods are the same, so this seems to be a trivial one-line fix.

I'll be happy to make the change if the issue is confirmed.

[bradfitz]

Yeah, feel free to send in a change. See https://golang.org/doc/contribute.html