Update reference to Stack version in documentation

Author: mpilgrem

doc/config_command.md

@@ -63,7 +63,7 @@ user-specific global configuration file (`config.yaml`).
 
 ## The `stack config set package-index download-prefix` command
 
-:octicons-tag-24: Unreleased
+:octicons-tag-24: 2.9.3
 
 ~~~text
 stack config set package-index download-prefix [--global] [URL]