man/io_uring_prep_cancel.3: correct io_uring_prep_cancel_fd() flags

For some reason this cancel variant takes an unsigned int - which is
arguably the correct type, but the others take just an int for the
flags. In any case, correct the man page to make it consistent with
the source code.

Closes: #1297
Signed-off-by: Jens Axboe <axboe@kernel.dk>

man/io_uring_prep_cancel.3

@@ -19,7 +19,7 @@ io_uring_prep_cancel \- prepare a cancelation request
 .PP
 .BI "void io_uring_prep_cancel_fd(struct io_uring_sqe *" sqe ","
 .BI "                          int " fd ","
-.BI "                          int " flags ");"
+.BI "                          unsigned int " flags ");"
 .fi
 .SH DESCRIPTION
 .PP