RDMA/rw: Fix rdma_rw_ctx_signature_init() kernel-doc header

Fixes: 0e353e34

 ("IB/core: add RW API support for signature MRs")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>