fix: use correct starting function for child_spec/1

[Gazler]

Summary of changes

Previously, the child_spec/1 function referenced a non-existent run function. This would mean that a child spec such as:

{HashRing.Managed, [name: :foo]}

Would not work, instead requiring the start function to be explicitly specified:

{HashRing.Managed, [name: :foo, start: {HashRing.Managed, :new, [:foo, []]}]}

This commit uses the new/2 function instead.

Checklist

• [X] New functions have typespecs, changed functions were updated
• [X] Same for documentation, including moduledocs
• [X] Tests were added or updated to cover changes
• [X] Commits were squashed into a single coherent commit

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• [ ] Notes added to CHANGELOG file which describe changes at a high-level

There is no changelog?