Update README.md (#221)

Fixing the README file. since the commit function is not using callback anymore

README.md

@@ -139,14 +139,14 @@ Move/rename a file from the `from` path to the `to` path.
 
 Returns `true` if a file exists. Returns `false` if the file is not found or deleted.
 
-### `#commit([filters,] [stream,] callback)`
+### `#commit([filters,] [stream,])`
 
 Persist every changes made to files in the mem-fs store to disk.
 
 If provided, `filters` is an array of TransformStream to be applied on a stream of vinyl files (like gulp plugins).
 If provided, `stream` is a stream of vinyl files.
 
-`callback` is called once the files are updated on disk.
+returns promise that is resolved once the files are updated on disk.
 
 ### `#dump([cwd,] [filter])`