[README.md] add ArtifactHub Badge (#3545)

* [README.md] add ArtifactHub Badge

Signed-off-by: junya koyama <arukiidou@yahoo.co.jp>

* [README.md] fix lint error

Signed-off-by: junya koyama <arukiidou@yahoo.co.jp>

* [README.md] fix lint error

Signed-off-by: junya koyama <arukiidou@yahoo.co.jp>

---------

Signed-off-by: junya koyama <arukiidou@yahoo.co.jp>

README.md

@@ -1,6 +1,6 @@
 # Prometheus Community Kubernetes Helm Charts
 
-[![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0) ![Release Charts](https://github.com/prometheus-community/helm-charts/workflows/Release%20Charts/badge.svg?branch=main) [![Releases downloads](https://img.shields.io/github/downloads/prometheus-community/helm-charts/total.svg)](https://github.com/prometheus-community/helm-charts/releases)
+[![Artifact Hub](https://img.shields.io/endpoint?url=https://artifacthub.io/badge/repository/prometheus-community)](https://artifacthub.io/packages/search?org=prometheus&cncf=true) [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0) ![Release Charts](https://github.com/prometheus-community/helm-charts/workflows/Release%20Charts/badge.svg?branch=main) [![Releases downloads](https://img.shields.io/github/downloads/prometheus-community/helm-charts/total.svg)](https://github.com/prometheus-community/helm-charts/releases)
 
 This functionality is in beta and is subject to change. The code is provided as-is with no warranties. Beta features are not subject to the support SLA of official GA features.
 
@@ -9,7 +9,7 @@ This functionality is in beta and is subject to change. The code is provided as-
 [Helm](https://helm.sh) must be installed to use the charts.
 Please refer to Helm's [documentation](https://helm.sh/docs/) to get started.
 
-Once Helm is set up properly, add the repo as follows:
+Once Helm is set up properly, add the repository as follows:
 
 ```console
 helm repo add prometheus-community https://prometheus-community.github.io/helm-charts