RFR: 8370912: Formatting error on JavaDoc because of missing quotation mark.

[iso53]

Corrected a missing closing quote in the HBox JavaDoc example, ensuring it's properly formatted.

-------------

Commit messages:
 - 8370912: Formatting error on JavaDoc because of missing quotation mark.

Changes: https://git.openjdk.org/jfx/pull/1956/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1956&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370912
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1956.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1956/head:pull/1956

PR: https://git.openjdk.org/jfx/pull/1956