RFR: JDK-8288740: Change incorrect documentation for sjavac flag

[Julian Waters]

The documentation for build performance currently points to the non-existent --enable-sjavac flag to enable sjavac, the correct one is actually --enable-javac-server (Finally seem to have gotten pandoc working, hopefully the html is correct this time)

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Commit messages:
 - Correct sjavac documentation in build docs

Changes: https://git.openjdk.org/jdk/pull/9215/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9215&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288740
  Stats: 5 lines in 2 files changed: 2 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/9215.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9215/head:pull/9215

PR: https://git.openjdk.org/jdk/pull/9215