Update prometheus.io's URL to the latest version (#5270)

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**

Co-Authored-By: Nguyen Phuong An <AnNP@vn.fujitsu.com>
Signed-off-by: Kim Bao Long <longkb@vn.fujitsu.com>

docs/index.md

@@ -2,12 +2,12 @@
 # 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