rhashtable: drop duplicated word in <linux/rhashtable.h>

Drop the doubled word "be" in a comment.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Thomas Graf <tgraf@suug.ch>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: netdev@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>