linux-next: DOC: RDS: Fix a typo in rds.txt

This patch fixes a spelling typo in rds.txt

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>