Update aria-required-owned to use gridcell in grid (#2137)

Feedback from the ARIA WG. Passed example 2 needs to use gridcell, not cell. This isn't required by the owned elements thing, but it is a MUST for the [grid role](https://w3c.github.io/aria/#grid).

Co-authored-by: Jean-Yves Moyen <jym@siteimprove.com>

Author: WilcoFiers

_rules/aria-required-owned-element-bc4a75.md

@@ -76,12 +76,12 @@ This element with the `list` role only owns elements with the `listitem` role. T
 
 #### Passed Example 2
 
-This element with the `grid` role only owns elements with the `row` role, and the element with the `row` role only owns elements with the `cell` role. The `row` role is one of the [required owned elements][] for `grid`, and `cell` is one of the [required owned elements][] for `row`.
+This element with the `grid` role only owns elements with the `row` role, and the element with the `row` role only owns elements with the `gridcell` role. The `row` role is one of the [required owned elements][] for `grid`, and `gridcell` is one of the [required owned elements][] for `row`.
 
 ```html
 <table role="grid">
 	<tr role="row">
-		<td role="cell">Item 1</td>
+		<td role="gridcell">Item 1</td>
 	</tr>
 </table>
 ```