Attribute typo in core-aam form-manual test

[jlp-craigmorten]

See: https://github.com/web-platform-tests/wpt/blob/master/core-aam/form-manual.html#L94

aria-labelled is an invalid attribute and should instead be aria-label.

This will potentially result in incorrect test results as the element will fall into the "form without an accessible name" classification instead of the (assumed) intended "form with an accessible name" classification.

REF:

• https://www.w3.org/TR/core-aam-1.2/#role-map-form
• https://www.w3.org/TR/core-aam-1.2/#role-map-form-nameless