Update README to replace references to theckman/go-flock
Update README to replace references to theckman/go-flock

This is one of the final steps needed to complete the migration to the Gofrs
organization.

Signed-off-by: Tim Heckman <8efd86fb78a56a5145ed7739dcb00c78581c5375@heckman.io>

gofrs/flock,theckman/go-flock