fix: update CLI URL in the installation instruction
fix: update CLI URL in the installation instruction

It currently points to a deprecated URL format we don't publish to
(starting from 1.13).

Eventually it will be replaced by a "latest" URL, but for now as DC/OS
1.13 is still in development we'll use the testing/master URL (the
latest commit).

Closes DCOS_OSS-4509

dcos/dcos-ui,dcos/dcos-ui,dcos/dcos-ui,dcos/dcos-ui,dcos/dcos-ui