RFR: 8358454: Wrong <br> tags in cssref.html

[Michael Strauß]

Some `<br>` tags in cssref.html are written as `<br/>` or `</br>`. It should be `<br>`, since it is a void element.

A single reviewer is sufficient.

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Commit messages:
 - fix wrong <br> HTML tags

Changes: https://git.openjdk.org/jfx/pull/1820/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1820&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358454
  Stats: 14 lines in 1 file changed: 0 ins; 0 del; 14 mod
  Patch: https://git.openjdk.org/jfx/pull/1820.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1820/head:pull/1820

PR: https://git.openjdk.org/jfx/pull/1820