RFR: 7903851: apidiff: typo in comment for HtmlDiffBuilderTest

[Jonathan Gibbons]

Please review a fix for a trivial typo in a comment in a test

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Commit messages:
 - CODETOOLS-7903851: apidiff: typo in comment for HtmlDiffBuilderTest

Changes: https://git.openjdk.org/apidiff/pull/21/files
  Webrev: https://webrevs.openjdk.org/?repo=apidiff&pr=21&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903851
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/apidiff/pull/21.diff
  Fetch: git fetch https://git.openjdk.org/apidiff.git pull/21/head:pull/21

PR: https://git.openjdk.org/apidiff/pull/21