Update GalleryAdmin.php

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | -
| Fixed tickets | 
| License       | MIT
| Doc PR        | -

Fix admin groups to prevent error
```
You should close previous group "Gallery" with end() before adding new tab "Options".
```