Fix another 'undefined index' notice and html typo

[HirotoKagotani]

The first commit fixes html typo </label to </label> in options-admin.php. The second one checks whether $shib_headers[*]['managed'] is set before use their value.

[michaelryanmcneill]

Hello, thank you for submitting this patch. I released version 1.8 today to resolve this and other issues and included a shoutout for your patch. I am the new maintainer of the plugin and all further work on the plugin will be done in a new GitHub repository. If you have any further issues, please don't hesitate to report them in the new repository.