RFR: JDK-8276800 Fix table headers in NumericShaper.html

[Ludvig Janiuk]

This change introduces no visual difference, but improves a11y.

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Commit messages:
 - Fix table headers in NumericShaper.html

Changes: https://git.openjdk.java.net/jdk/pull/6291/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6291&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276800
  Stats: 8 lines in 1 file changed: 0 ins; 3 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6291.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6291/head:pull/6291

PR: https://git.openjdk.java.net/jdk/pull/6291