Fix binary search.

Finetune a figure in intro_to_dp.

codes/python/chapter_searching/binary_search_edge.py

@@ -32,7 +32,7 @@ def binary_search_right_edge(nums: list[int], target: int) -> int:
             j = m - 1  # target 在区间 [i, m-1] 中
         else:
             i = m + 1  # 首个大于 target 的元素在区间 [m+1, j] 中
-    if j == len(nums) or nums[j] != target:
+    if j < 0 or nums[j] != target:
         return -1  # 未找到目标元素，返回 -1
     return j