RFR: 7903834: apidiff: DocPath issue on Windows

[Jonathan Gibbons]

Please review a small fix to correctly handle Windows paths.

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Commit messages:
 - CODETOOLS-7903834: apidiff: DocPath issue on Windows

Changes: https://git.openjdk.org/apidiff/pull/8/files
  Webrev: https://webrevs.openjdk.org/?repo=apidiff&pr=8&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903834
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/apidiff/pull/8.diff
  Fetch: git fetch https://git.openjdk.org/apidiff.git pull/8/head:pull/8

PR: https://git.openjdk.org/apidiff/pull/8