coverage: use shields.io to serve coverage status badge
coverage: use shields.io to serve coverage status badge

This seems to fix the color issue where it was
already red despite it not being below the failure threshold.

Signed-off-by: Noah Watkins <86ecd7d1c53cf33a85014f7cab78876cb4491020@gmail.com>
noahdesu/zlog,abhigrover101/zlog,abhigrover101/zlog,abhigrover101/zlog,noahdesu/zlog,noahdesu/zlog,noahdesu/zlog,noahdesu/zlog,abhigrover101/zlog,abhigrover101/zlog