docs: arm: tcm: Fix typo in description of TCM and MMU usage

Correct a typo in the description of interaction between
the TCM and MMU.

Found by inspection.

Signed-off-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20220609184230.627958-1-simon.horman@corigine.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>