docs: use correct env var name for cache cleanup (#125)

Signed-off-by: Sergei Lukianov <me@slukjanov.name>

Author: Frostman

docs/content/1.getting-started/1.index.md

@@ -58,7 +58,7 @@ variant: subtle
 ---
 ::
 
-#### `CLEANUP_OLDER_THAN_DAYS`
+#### `CACHE_CLEANUP_OLDER_THAN_DAYS`
 
 - Default: `90`