RFR: 6441827: Documentation mentions nonexistent NullReferenceException

[Jaikiran Pai]

Can I please get a review of this doc only change which fixes a trivial typo in the documentation of `ObjectOutputStream.replaceObject()` method, as noted in https://bugs.openjdk.org/browse/JDK-6441827?

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Commit messages:
 - 6441827: Documentation mentions nonexistent NullReferenceException

Changes: https://git.openjdk.org/jdk/pull/13399/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13399&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-6441827
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13399.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13399/head:pull/13399

PR: https://git.openjdk.org/jdk/pull/13399