RFR: 8320199: Fix HTML 5 errors in java.math.BigInteger

[Brian Burkhalter]

Clean up HTML error due to nested anchor (`<a>`) elements.

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Commit messages:
 - 8320199: Fix HTML 5 errors in java.math.BigInteger

Changes: https://git.openjdk.org/jdk/pull/16679/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16679&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320199
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/16679.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16679/head:pull/16679

PR: https://git.openjdk.org/jdk/pull/16679