rename `findDescendants` to `findDescendant` (#36249)
rename `findDescendants` to `findDescendant` (#36249)

Slate doesn't have a `findDescendants` method on `BaseNode` but `findDescendant`.
georgemarshall/DefinitelyTyped,markogresak/DefinitelyTyped,mcliment/DefinitelyTyped,georgemarshall/DefinitelyTyped,dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,dsebastien/DefinitelyTyped