Reword method doc block

src/Cache/SimpleCacheEngine.php

@@ -57,7 +57,7 @@ protected function ensureValidKey($key)
     }
 
     /**
-     * Fetches a value from the cache.
+     * Fetches the value for a given key from the cache.
      *
      * @param string $key The unique key of this item in the cache.
      * @param mixed $default Default value to return if the key does not exist.