profile-manual: usage.rst: formatting fixes

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

(From yocto-docs rev: f0c7c67ad297588d0112070b82f46fbc779bcb14)

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