fix Duplicate `Adding sidebar menu item` if installed multiple times

[egyjs]

issue

In windows, I installed Laravel-Backpack/FileManager , then called this command:

 php artisan backpack:filemanager:install

then uploaded the script to a Linux server and again ran php artisan backpack:filemanager:install, and

the item added again in the sidebar..

i opened `` and compare them

and this is the result:

+ <li class="nav-item"><a class="nav-link" href="{{ backpack_url('elfinder') }}"><i class="nav-icon la la-files-o"></i> <span>{{ trans('backpack::crud.file_manager') }}</span></a></li>
- <li class="nav-item"><a class="nav-link" href="{{ backpack_url('elfinder') }}\"><i class="nav-icon la la-files-o"></i> <span>{{ trans('backpack::crud.file_manager') }}</span></a></li>