Replace SIGHUP example in docs with one that doesn't assume arcane Unix knowledge

[njsmith]

Currently in our docs for open_signal_receiver, we have an example of catching SIGHUP and using it to reload the application configuration. While talking to @wgwz today re: python-trio/snekomatic#41, I discovered that this example is maybe not so great, because readers who aren't already steeped in Unix traditions will have no idea what the heck a SIGHUP is supposed to be.

Using SIGTERM in the example would probably be less distracting!