prom: try to fix links in cheat sheet, again, again

Turns out we had a missing paren, but also it seems like just having
the en-encoded parens, *just for this link*, broke things.

Markdown parsers, go figure.

service/prometheus.md

@@ -1032,7 +1032,7 @@ See also the [CPU][], [memory][], and [disk][] dashboards.
 
 [Number of machines]: https://prometheus.torproject.org/graph?g0.expr=count(up{job%3D"node"})
 [Number of machine per OS version]: https://prometheus.torproject.org/graph?g0.expr=count(node_os_info)+by+(version_id,+version_codename)
-[Number of machines per exporters]: https://prometheus.torproject.org/graph?g0.expr=sort_desc(sum(up{job%3D~"$job"})+by+(job)
+[Number of machines per exporters]: https://prometheus.torproject.org/graph?g0.expr=sort_desc%28sum%28up%7Bjob%3D~%22$job%22%7D%29+by+%28job%29%29
 [Number of CPU cores, memory size, file system and LVM sizes]: https://prometheus.torproject.org/graph?g0.expr=count(node_cpu_seconds_total{classes%3D~"$class",mode%3D"system"})
 [Uptime, in days]: https://prometheus.torproject.org/graph?g0.expr=round((time()+-+node_boot_time_seconds)+/+(24*60*60))
 [CPU]: https://grafana.torproject.org/d/gex9eLcWz/cpu-usage