Update 4.shellSort.md

Author: juliofeng

4.shellSort.md

@@ -69,7 +69,7 @@ def shellSort(arr):
 func shellSort(arr []int) []int {
 length := len(arr)
 gap := 1
-for gap < gap/3 {
+for gap < length/3 {
 gap = gap*3 + 1
 }
 for gap > 0 {