Rename service unit file to match package name

So that debhelper automatically installs it for me, instead of adding my
own install target.

debian/another-boids-in-rust.service

@@ -0,0 +1,14 @@
+[Unit]
+Description=A website hosting a WASM build of Boids
+After=network.target
+
+[Service]
+Type=simple
+ExecStart=/usr/bin/env python3 -m http.server -d /usr/share/boids 8000
+Restart=never
+User=nobody
+Group=nobody
+WorkingDirectory=/usr/share/boids
+
+[Install]
+WantedBy=multi-user.target