profile-manual: usage.rst: formatting fixes

Plus a few text styling improvements, some reported by "make stylecheck"

(From yocto-docs rev: ce0e83716197773d8eae0c2f0edc1cf290ebd60f)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>