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

[Jonathan Gibbons]

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

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Commit messages:
 - CODETOOLS-7903823

Changes: https://git.openjdk.org/apidiff/pull/6/files
  Webrev: https://webrevs.openjdk.org/?repo=apidiff&pr=6&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903823
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/apidiff/pull/6.diff
  Fetch: git fetch https://git.openjdk.org/apidiff.git pull/6/head:pull/6

PR: https://git.openjdk.org/apidiff/pull/6