Fix recomputeTree example in README.md

`subtreeCallback` was wrongly identified.

Author: ptoussai

README.md

@@ -278,7 +278,7 @@ const tree = {
 tree.recomputeTree({
   'root-1': {
     open: false,
-    subtreeWalker(node, ownerNode) {
+    subtreeCallback(node, ownerNode) {
       // Since subtreeWalker affects the ownerNode as well, we can check if the
       // nodes are the same, and run the action only if they aren't
       if (node !== ownerNode) {
@@ -659,4 +659,4 @@ const Node = ({data: {isLeaf, name}, isOpen, style, setOpen}) => (
 
 ### 5. Migrate all your IDs to string
 
-Using node IDs as keys should improve React rendering performance. However, it means that you won't be able to use `Symbol` as IDs anymore. You should move all your IDs to be strings instead of symbols.
+Using node IDs as keys should improve React rendering performance. However, it means that you won't be able to use `Symbol` as IDs anymore. You should move all your IDs to be strings instead of symbols.