cpumask: align text in comment

Since commit 4e1a7df4 ("cpumask: Add enabled cpumask
for present CPUs that can be brought online") introduced
cpu_enabled_mask, the comment line describing the mask
has been slightly out of alignment with the adjacent
lines.

Fix this by removing a single space character.

Signed-off-by: Joel Savitz <jsavitz@redhat.com>
Signed-off-by: Yury Norov <yury.norov@gmail.com>