README.md: fix the Markdown
README.md: fix the Markdown

Markdown requires that a codeblock starts on a paragraph of its own,
i.e, with a preceding empty line.  Fix it so that the assets directory
is shown as code in github.

credp/lisa,bjackman/lisa,credp/lisa,ARM-software/lisa,JaviMerino/lisa,joelagnel/lisa,bjackman/lisa,arnoldlu/lisa,arnoldlu/lisa,mdigiorgio/lisa,mdigiorgio/lisa,ARM-software/lisa,credp/lisa,joelagnel/lisa,credp/lisa,ARM-software/lisa,ARM-software/lisa