RFR: JDK-8294536: Update troff form of man page for new --spec-base-url option

[Jonathan Gibbons]

Please review a trivial change to update the open form of the javadoc man page.

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Commit messages:
 - JDK-8294536: Update troff form of man page for new --spec-base-url option

Changes: https://git.openjdk.org/jdk/pull/10972/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10972&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294536
  Stats: 10 lines in 2 files changed: 9 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/10972.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10972/head:pull/10972

PR: https://git.openjdk.org/jdk/pull/10972