docs: Fix indentation

Additional paragraphs in a rST list should be indented by the same
number of characters as the first paragraph otherwise it renders as a
block quote. Correct this for the backwards incompatible changes guide.

Change-Id: I918c2a24aead4cc5a317201df9b12ce740612e5a
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

Author: stephenfin