docs/mm: concepts.rst: remove unnecessary word

Signed-off-by: Bhaskar Chowdhury <unixbhaskar@gmail.com>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Link: https://lore.kernel.org/r/20200721112251.6100-1-unixbhaskar@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>