Point to an active fork of Fleet

[iansltx]

Kolide retired their original Fleet repo years back, and has pulled source from the current commit of the repo due to vulnerabilities found since then. Revising the link here to point to, to my knowledge, the most active fork of Fleet (the one at https://fleetdm.com), which can still be used as an osquery data sink, including in its MIT-licensed/free form.

Full disclosure: I work for Fleet.