Update inaccurate mapChildren comment (#18931)

The function you provide will only be passed a child and an index. It will not be passed a key. This is confirmed in the source, the Flow types, and the jsdoc comments.

packages/react/src/ReactChildren.js

@@ -212,7 +212,7 @@ type MapFunc = (child: ?React$Node) => ?ReactNodeList;
  *
  * See https://reactjs.org/docs/react-api.html#reactchildrenmap
  *
- * The provided mapFunction(child, key, index) will be called for each
+ * The provided mapFunction(child, index) will be called for each
  * leaf child.
  *
  * @param {?*} children Children tree container.