Fix Scenic.Graph.find/2 docs and spec

[axelson]

Description

The docs and spec were wrong about the return type of the function

Types of changes

• [ ] Bug fix (a non-breaking change which fixes an issue)
• [ ] New feature (a non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] Improvement/refactoring (non-breaking change that doesn't add any feature but make things better)

Checklist

• [x] Check other PRs and make sure that the changes are not done yet.
• [x] The PR title is no longer than 64 characters.