docs:mm: fix spelling mistakes in heterogeneous memory management page

Signed-off-by: Dennis Lam <dennis.lamerice@gmail.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20240908161928.3700-1-dennis.lamerice@gmail.com>