docs: Fix formatting of security document
docs: Fix formatting of security document

Markdown considers a block of indented text after a list item to be part
of that list item.  Insert a small piece of text at the top level to
prevent that.

Signed-off-by: David Brown <0af22db655f3ade5845314d695c1f4910314b0cb@linaro.org>

utzig/mcuboot,runtimeco/mcuboot,utzig/mcuboot,runtimeco/mcuboot,tamban01/mcuboot,utzig/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,utzig/mcuboot,runtimeco/mcuboot,utzig/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,tamban01/mcuboot