Replaced "role=contentinfo" with "role=banner" in "Rules of ARIA attribute usage by HTML element" table (header element)

[giacomo-petri]

Closes #501

• 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.)

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[w3cbot]

scottaohara marked as non substantive for IPR from ash-nazg.