add a little space above h2->h6 - it gets a bit dense in there when there are lots of steps

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@home.org.au> (github: SvenDowideit)
Merge pull request #4173 from SvenDowideit/add-most-spacing-to-headers

add a little space above h2->h6 - it gets a bit dense in there when there are lots of steps