ch4: fix typo in "Tree Traversal Using Generators" (#66)

"when we can’t we move" -> "when we can’t move"

Author: vsemozhetbyt

ch04.asciidoc

@@ -1376,7 +1376,7 @@ class Node {
 }
 ----
 
-Trees can be traversed using depth-first search, where we always try to go deeper into the tree structure, and when we can't we move to the next children on the list. In the following tree structure, a depth-first search algorithm would traverse the tree visiting the nodes following the `1, 2, 3, 4, 5, 6, 7, 8, 9, 10` order.
+Trees can be traversed using depth-first search, where we always try to go deeper into the tree structure, and when we can't move to the next children on the list. In the following tree structure, a depth-first search algorithm would traverse the tree visiting the nodes following the `1, 2, 3, 4, 5, 6, 7, 8, 9, 10` order.
 
 [source,javascript]
 ----