docs: fix undefined variable in caching example

Author: CybotTM

docs/developers/caching.rst

@@ -64,7 +64,7 @@ You can use any PSR-16 compatible cache implementation:
     use Symfony\Component\Cache\Adapter\FilesystemAdapter;
     use Symfony\Component\Cache\Psr16Cache;
 
-    $pool = new FilesystemAdapter('inventory', 3600, $cacheDir);
+    $pool = new FilesystemAdapter('inventory', 3600, '/path/to/cache');
     $cache = new Psr16Cache($pool);
 
 **Redis Cache** (for shared/distributed caching):