Update PROMETHEUS.md

Signed-off-by: Webb Brown <298359+dwbrown2@users.noreply.github.com>
Signed-off-by: Kaelan Patel <kaelanspatel@gmail.com>

PROMETHEUS.md

@@ -1 +1 @@
-<https://www.opencost.io/docs/>
+Available at <https://www.opencost.io/docs/prometheus>