chore: add a link to Immer in the docs

Author: Fannolo

docs/api/createSlice.mdx

@@ -15,7 +15,7 @@ and automatically generates action creators and action types that correspond to
 This API is the standard approach for writing Redux logic.
 
 Internally, it uses [`createAction`](./createAction.mdx) and [`createReducer`](./createReducer.mdx), so
-you may also use Immer to write "mutating" immutable updates:
+you may also use [Immer](https://immerjs.github.io/immer/) to write "mutating" immutable updates:
 
 ```ts
 import { createSlice, PayloadAction } from '@reduxjs/toolkit'