DOC: config: use the word 'backend' instead of 'proxy' in 'track' description

DOC: config: use the word 'backend' instead of 'proxy' in 'track' description

User @nwehrman reported in issue #2328 that the used of "proxy" instead
of "backend" in the argument of the "track" server keyword is confusing.
Admittedly, all other places in the doc use "backend/server" instead of
"proxy/server", so let's update it for the sake of consistency.