docs: Fix infiniband uverbs minor number

Starting from the beginning of infiniband subsystem, the uverbs char
devices start from 192 as a minor number, see
commit bc38a6ab ("[PATCH] IB uverbs: core implementation").

This patch updates the admin guide documentation to reflect it.

Fixes: 9d85025b ("docs-rst: create an user's manual book")
Link: https://lore.kernel.org/r/bad03e6bcde45550c01e12908a6fe7dfa4770703.1627477347.git.leonro@nvidia.com


Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>