scripts: ci: check_compliance: improve the "not sorted" command

Add a message to the KeepSorted error message with a oneline command to
sort the block failing the check.

Looks something like:

KeepSorted:sorted block is not sorted, sort by running:
"ex -s -c '14,757 sort i|x' dts/bindings/vendor-prefixes.txt"

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>