docs(prerequisites): fix "Build Host" title

Add an empty line just before the "Build Host" title.

Without this, the title is not properly recognized, it does not get
added to the table of contents and the underlining characters appear
as dashes, as can be seen here:

https://trustedfirmware-a.readthedocs.io/en/v2.7/getting_started/prerequisites.html#prerequisites



Change-Id: Ia89cf3de0588495cbe64b0247dc860619f5ea6a8
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>