format-code: fail on markdownlint warnings

We've cleaned up the vast majority of markdownlint warnings and the
few remaining have pending commits.  Enable failures on markdownlint
warnings now so we do not regress.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I3e16bd65cf0b3aefc818b027c8b368dc56819332
1 file changed