boards nrf52_bsim: Fix sample used as example

Let's fix the sample used in the example. The peripheral_hr and
central_hr are meant to be run with each other.
Let's also use the :zephyr:code-sample: directive to refer to the
samples so we get a link.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>