docs(core): fix panic section on manage documentation

[lucasfernog]

What kind of change does this PR introduce?

• [ ] Bugfix
• [ ] Feature
• [x] Docs
• [ ] New Binding issue #___
• [ ] Code style update
• [ ] Refactor
• [ ] Build-related changes
• [ ] Other, please describe:

Does this PR introduce a breaking change?

• [ ] Yes, and the changes were approved in issue #___
• [ ] No

Checklist

• [ ] When resolving issues, they are referenced in the PR's title (e.g fix: remove a typo, closes #___, #___)
• [ ] A change file is added if any packages will require a version bump due to this PR per the instructions in the readme.
• [ ] I have added a convincing reason for adding this feature, if necessary

Other information

[FabianLars]

Am i missing another related change here? Because it does panic if it's already managed.

Edit: Talking about the Builder, not the Manager trait.

[lucasfernog]

Damn I totally missed the assert in the Builder and only tested the Manager trait. Thanks for catching it.