docs/install-tutorial.rst: add proxy_set_header
docs/install-tutorial.rst: add proxy_set_header

proxy_set_header Host is required so flask knows the hostname it should respond to when being requested via a socket.
not doing so results in redirects to \`localhost\` when trying add new pastes

bepasty/bepasty-server,bepasty/bepasty-server,bepasty/bepasty-server,bepasty/bepasty-server