Fix wrong code example in the UPGRADE-4.0.md file (#416)

Author: araab

UPGRADE-4.0.md

@@ -66,7 +66,7 @@ using an on-premise installation it requires Sentry version `>= v20.6.0` to work
   sentry:
       options:
           integrations:
-              - '@Sentry\Integration\IgnoreErrorsIntegration'
+              - 'Sentry\Integration\IgnoreErrorsIntegration'
 
   services:
       Sentry\Integration\IgnoreErrorsIntegration: