Update extensions.md (#949)

Fixed broken link to Percona Support for ver 17

Author: kavants

docs/extensions.md

@@ -10,7 +10,7 @@ Percona Distribution for PostgreSQL includes the extensions that have been teste
 
 Percona also supports [extra modules](https://repo.percona.com/ppg-17-extras/), not included in Percona Distribution for PostgreSQL but tested to work with it.
 
-Additionally, see the list of [PostgreSQL software](https://www.percona.com/services/support/support-tiers-postgresql) covered by Percona Support.
+Additionally, see the list of [PostgreSQL software](https://www.percona.com/postgresql/support/) covered by Percona Support.
 
 ## Install an extension