crypto: rng - fix documentation for crypto_rng_alg()

Current documentation states that crypto_rng_alg() returns the cra_name of
the rng algorithm, but it actually returns a 'struct rng_alg' pointer from
a RNG handle.

Update documentation to reflect this.

Signed-off-by: Ovidiu Panait <ovidiu.panait.oss@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>