[css-gcpm] 'element value()' should be 'element() value'

[tgraham-antenna]

csswg-drafts/css-gcpm-3/Overview.bs

Line 306 in 0d6e1a1

<p class="note">The element ''value()'' can only be used in page margin boxes. And it cannot be combined with other possible values for the content property.</p>

element() is a value, value() is not an element.