Documentation/admin-guide: fixes for thunderbolt.rst

Edits for grammar, punctuation, and a doubled-up word.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Andreas Noever <andreas.noever@gmail.com>
Cc: Michael Jamet <michael.jamet@intel.com>
Cc: Mika Westerberg <mika.westerberg@linux.intel.com>
Cc: Yehezkel Bernat <yehezkel.bernat@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>