Upd readme

Author: igorwojda

src/test/kotlin/com/igorwojda/tree/classic/levelwidth/README.md

@@ -6,7 +6,7 @@
 
 ## Instructions
 
-Given the root node of a tree, return an list where each element is the width of the tree at each level.
+Given the root node of a tree, return a list where each element is the width of the tree at each level.
 
 [Challenge](Challenge.kt) | [Solution](Solution.kt)