Change link from lead-free to lead-safe

[geneorama]

The icon at the top of the page points to lead-free, not lead-safe
http://dev.cityofchicago.org/docs/lead-safe/

content is here: https://github.com/Chicago/lead-safe-api-docs/blob/1d18fc5fdc307a224d48e9a674e0b4d428e73f5f/mkdocs.yml