RFR: 7903823: Use correct key when building display of method return types

[Iris Clark]

On Wed, 18 Sep 2024 20:27:48 GMT, Jonathan Gibbons <jjg at openjdk.org> wrote:

> Please review a simple fix to use the correct position-key to identify the different return types of a method.

Marked as reviewed by iris (Reviewer).

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PR Review: https://git.openjdk.org/apidiff/pull/6#pullrequestreview-2313762610