docs, kprobes: Add riscv as supported architecture

Support of kprobes and kretprobes for riscv was introduced 3 years ago
by the following change:

commit c22b0bcb ("riscv: Add kprobes supported")

Add riscv to the list of supported architectures.

Signed-off-by: Ivan Orlov <ivan.orlov@codethink.co.uk>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240429155735.68781-1-ivan.orlov@codethink.co.uk