Update predefined segments link in Configuring Jetstream docs

There's a link in the Experiment Section section to "mozanalysis documentation" that is broken and also out of date. We have segments listed in metric-hub docs. The current link is [mozanalysis documentation](https://mozilla.github.io/mozanalysis/api/segments/desktop.html) but this should instead be [metric-hub documentation](https://mozilla.github.io/metric-hub/segments/firefox_desktop/).