Wrong role for Header in "Rules of ARIA attribute usage by HTML element" table

[giacomo-petri]

• URL: https://www.w3.org/TR/html-aria/#el-header
• Issue description:
  The existing content in the "ARIA role, state, and property allowances" cell for the Header element wrongly specify role=contentinfo instead of role=banner (as follows):

  Roles: group, none or presentation. (If not a descendant of an article, aside, main, nav or section element, or an element with role=article, complementary, main, navigation or region, then role=contentinfo is also allowed, but NOT RECOMMENDED. Otherwise, role=generic is also allowed, but SHOULD NOT be used.)

I am submitting a pull request that resolve the issue.