dev-manual: Spell check.

Found a couple words that were fat-fingered and fixed them.

(From yocto-docs rev: 593fd043f350bbce302c3de7dce0ab4bdbd2f247)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>