Update prometheus.io's URL to the latest version
Currently, the lastest version is **2.7**. But the version in web page is **2.0**.
So this commit aims to update the URL to the latest version of **prometheus.io**
# todo: internal
---
-# Prometheus 2.0
+# Prometheus
Welcome to the documentation of the Prometheus server.
The documentation is available alongside all the project documentation at
-[prometheus.io](https://prometheus.io/docs/prometheus/2.0/).
+[prometheus.io](https://prometheus.io/docs/prometheus/latest/).
## Content
