Fix table headers in NumericShaper.html

java.desktop/java/awt/font/NumericShaper.html does not have proper table headers, fix to improve a11y.

More specifically, Doccheck [1] flagged it, my understanding is that every row needs to have a unique row header to aid screen readers. The previous code had one cell work as header for two rows.

One could perhaps look at the standards [2] and implement something even better, but this is what I came up with.

[1]: https://openjdk.java.net/projects/code-tools/doccheck/
[2]: https://html.spec.whatwg.org/multipage/tables.html#attr-tdth-headers