docs: updated tagline in readme (#244831)

## Summary

We launched a revision to our messaging on the
https://www.elastic.co/kibana page. This PR aligns the README.md to that
wording. The tagline for Kibana has already been updated (thanks
@lukeelmers )

Author: ghudgins

README.md

@@ -1,6 +1,6 @@
 # Kibana
 
-Kibana is the window into the [Elastic Stack](https://www.elastic.co/products). Specifically, it's a browser-based analytics and search dashboard for Elasticsearch.
+Kibana is the open source interface to query, analyze, visualize, and manage your data stored in Elasticsearch.
 
 - [Getting Started](#getting-started)
   - [Using a Kibana Release](#using-a-kibana-release)