RFR: 8371009: HttpClient javadoc synchronous example missing HttpRequest variable declaration

[EunHyunsu]

Fixes JDK-8371009.

The synchronous HttpClient example references `request` without defining it.
This patch adds a minimal HttpRequest initialization immediately before the
synchronous example (mirroring the asynchronous snippet) so the example
compiles and runs as-is.

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Commit messages:
 - JDK-8371009: HttpClient javadoc synchronous example missing HttpRequest variable declaration

Changes: https://git.openjdk.org/jdk/pull/28084/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28084&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371009
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/28084.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28084/head:pull/28084

PR: https://git.openjdk.org/jdk/pull/28084