Documentation/ABI: Correct mlxreg-io KernelVersion for 5.0

The mlxreg-io for the merge window assumed 4.21 as the next kernel
version. Replace 4.21 with 5.0.

Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>