crypto/tls: update ExampleDial with a valid root

crypto/tls: update ExampleDial with a valid root

When run as a separate program, the code in ExampleDial panicked due to
an expired certificate. Fixed this problem by replacing the expired
certificate with a valid one.

Also added a comment in the certificate to give a hint about why it
might fail in the future.

Fixes #35706

Change-Id: I3d300f7bccae050e4b73ded28b8029aa04b480bd
Reviewed-on: https://go-review.googlesource.com/c/go/+/212601
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>