Fix a typo

`belongsToMany` operation is imporing the `Model` with a typo. Fixed it.

README.md

@@ -751,7 +751,7 @@ The belongsToMany relation will not use a pivot "table" but will push id's to a
 If you want to define custom keys for your relation, set it to `null`:
 
 ```php
-use Jenssegers\Mongodb\Eloquent\Mode;
+use Jenssegers\Mongodb\Eloquent\Model;
 
 class User extends Model
 {