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>
(cherry picked from commit a34dcba9ebcda5a1dcb95b332486454776cd305b)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/customize-directory-structure.txt

@@ -27,10 +27,10 @@ to you.
 |           +-- post_image.sh
 |           +-- rootfs_overlay/
 |           |   +-- etc/
-|           |   +-- <some file>
+|           |   +-- <some files>
 |           +-- patches/
 |               +-- foo/
-|               |   +-- <some patch>
+|               |   +-- <some patches>
 |               +-- libbar/
 |                   +-- <some other patches>
 |