Fix mapFunction parameters in mapChildren's JSDoc (#6569)

Remove 'key' parameter from 'mapFunction(child, key, index)'. It is no longer called with 'key'.

src/isomorphic/children/ReactChildren.js

@@ -146,7 +146,7 @@ function mapIntoWithKeyPrefixInternal(children, array, prefix, func, context) {
 /**
  * Maps children that are typically specified as `props.children`.
  *
- * 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.