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>

Documentation/hw_random.txt

@@ -63,7 +63,7 @@ Intel RNG Driver notes:
 
 	* FIXME: support poll(2)
 
-	NOTE: request_mem_region was removed, for two reasons:
+	NOTE: request_mem_region was removed, for three reasons:
 	1) Only one RNG is supported by this driver, 2) The location
 	used by the RNG is a fixed location in MMIO-addressable memory,
 	3) users with properly working BIOS e820 handling will always