hwrng: Fix a wrong comment in Documentation/hw_random.txt

Seeing from the comment, there should be three reasons for removing request_mem_region.
Change the comment "two" to "three".

Signed-off-by: Tang Chen <tangchen@cn.fujitsu.com>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>