Fix typo in the runtime configuration example (cortexproject#3663)

The mirror enabled option uses `mirror_enabled` as key as you can see [here](https://github.com/cortexproject/cortex/blob/6165acfd5dcfc99aca631fce2601b2e6e663c12f/pkg/ring/kv/multi.go#L72)

Signed-off-by: Mario de Frutos <mario@defrutos.org>

Author: ethervoid

docs/configuration/arguments.md

@@ -416,7 +416,7 @@ overrides:
     max_series_per_query: 100000
 
 multi_kv_config:
-    mirror-enabled: false
+    mirror_enabled: false
     primary: memberlist
 ```