Update index.md (#955)

Updated link to PMM documentation from PMM 2 which is EOL to PMM 3

Author: kavants

docs/index.md

@@ -14,7 +14,7 @@ Part of the solution, Percona Operator for PostgreSQL, makes it easy to orchestr
 - No guesswork on finding the right version of a component – they all undergo thorough testing to ensure compatibility
 - Freely available reference architectures for solutions like high-availability, backups and disaster recovery 
 - Spatial data handling support via PostGIS
-- Monitoring of the database health, performance and infrastructure usage via open source [Percona Management and Monitoring :octicons-link-external-16:](https://www.percona.com/doc/percona-monitoring-and-management/2.x/index.html) with PostgreSQL-specific dashboards
+- Monitoring of the database health, performance and infrastructure usage via open source [Percona Management and Monitoring :octicons-link-external-16:](https://docs.percona.com/percona-monitoring-and-management/3/) with PostgreSQL-specific dashboards
 - Run PostgreSQL on Kubernetes using open source [Percona Operator for PostgreSQL :octicons-link-external-16:](https://docs.percona.com/percona-operator-for-postgresql/2.0/index.html). It not only automates deployment and management of PostgreSQL clusters on Kubernetes, but also includes enterprise-ready features for high-availability, backup and restore, replication, logging, and more 
 - Automate PostgreSQL provisioning and management in Kubernetes, in the cloud or on-premises with [Percona Everest :octicons-link-external-16:](https://docs.percona.com/everest/index.html#why-percona-everest) - the cloud-native database platform