Editorial - Removed "or" at the beginning of the list of allowed roles for "form" element

[giacomo-petri]

• URL: https://www.w3.org/TR/html-aria/#el-form

• Issue description: In "Rules of ARIA attribute usage by HTML element" table, the "form" allowed explicit roles cell starts with an "or":

  Roles: or none, presentation or search. (form is also allowed, but NOT RECOMMENDED.)

I am submitting a pull request that resolve the typo.