RFR: 8260596: Comment cleanup in BigInteger

[Weijun Wang]

Fix some small errors.

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Commit messages:
 - 8260596: Comment cleanup in BigInteger

Changes: https://git.openjdk.java.net/jdk/pull/2292/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2292&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260596
  Stats: 5 lines in 1 file changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2292.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2292/head:pull/2292

PR: https://git.openjdk.java.net/jdk/pull/2292