RDMA: rdma_user_ioctl.h: fix a duplicated word + clarify

Change the repeated word "it" in a comment to "it to".
Also insert a dash in the sentence to add clarity.

Link: https://lore.kernel.org/r/20200719003220.21250-1-rdunlap@infradead.org


Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>