docs/manual: fixed some spelling mistakes

Fixed some spelling mistakes of countable nouns.

Signed-off-by: Zikui Zhao <zhaozikui@eswincomputing.com>
Reviewed-by: Woodrow Douglass <wdouglass@carnegierobotics.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>