doc: fix grammar

Some minor typos:

  - make is unbindable -> make it unbindable
  - a underlying -> an underlying
  - different version -> different versions

Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Javi Merino <javi.merino@arm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>