docs: proc.rst: mountinfo: improved field numbering

Without reading thoroughly, one could easily oversee that there may be several
fields after #6.
Made it more clearly by changing the field numbering.

Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>
Link: https://lore.kernel.org/r/20211007040001.103413-2-mail@christoph.anton.mitterer.name
Signed-off-by: Jonathan Corbet <corbet@lwn.net>