<AWT Dev> RFR: 8260462: Missing <thead> in Modality.html

[Alexey Ivanov]

The first row in The Standard Blocking Matrix table in The AWT Modality (`Modality.html`) is a header row and it should be inside `<thead>` element.

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Commit messages:
 - 8260462: Missing <thead> in Modality.html

Changes: https://git.openjdk.java.net/jdk/pull/2314/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2314&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260462
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2314.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2314/head:pull/2314

PR: https://git.openjdk.java.net/jdk/pull/2314