fix document.getPath with node key as input in findPath util function

[kwokhuen]

Is this adding or improving a feature or fixing a bug?

Fixing a bug.

What's the new behavior?

In slate-react/utils, findPath function does not work as expected, as document.getPath function required a node key as input.

How does this change work?

document.getPath should take node key as input.

Have you checked that...?

• The new code matches the existing patterns and styles.
• The tests pass with yarn test.
• The linter passes with yarn lint. (Fix errors with yarn prettier.)
• The relevant examples still work. (Run examples with yarn watch.)

Does this fix any issues or need any specific reviewers?

Fixes: #
Reviewers: @ianstormtaylor

[ianstormtaylor]

As of #3093 (which was just merged), I believe this issue is no longer applicable, because a lot has changed. I'm going through and closing out any potential issues that are not out of date with the overhaul. Thanks for understanding.