dt-bindings: ASoc: Fix tdm-slot documentation spelling error

Fix the spelling of 'specified'.  Also fix grammarical issue with the
use of 'a' over 'an'

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Link: https://lore.kernel.org/r/20200602193524.30309-1-dmurphy@ti.com
Signed-off-by: Mark Brown <broonie@kernel.org>