hwrng: doc - Fix device node name reference /dev/hw_random => /dev/hwrng

hwrng: doc - Fix device node name reference /dev/hw_random => /dev/hwrng

In April 2009, commit d405640 ("Driver Core: misc: add node name support
for misc devices.") inadvertently changed the device node name from
/dev/hw_random to /dev/hwrng.  Since 6 years has passed since the change
it seems unpractical to change it back, as this node name is probably
considered ABI by now.  So instead, we'll just change the documentation
to match the current situation.

NB: It looks like rng-tools have already been updated.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Kieran Bingham <kieranbingham@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>