fix: remove duplicate insertion to visited set

[Lai-YT]

There is a subtle mistake in the Python3 version of the algorithm: the point (x, y) is inserted into the visited (vis) set twice.
The implementation and both versions of the documentation are corrected in this pull request.

Note

This isn't really a bug, as the algorithm itself remains correct 😆 .

I've also submitted the modified solution to LeetCode, and it successfully passed all the tests!

[idoocs]

🤭 感谢你的提交，请检查你的改动是否符合以下项目规范。

1. 格式化

我们项目中各种编程语言代码（包括文档）所采用的格式化工具不同，提交 pr 之前必须确保代码、文档正确格式化。

• .{md,js,ts,php,sql} 采用 prettier
• .{c,cpp,java} 采用 clang-format
• .{py} 采用 black
• .{go} 采用 gofmt
• 其它待完善

2. Git 提交信息

我们项目遵循 AngularJS Git Commit Message Conventions 规范，我们希望你的提交信息尽可能与项目保持一致。

• 新增或修改题解：feat: add/update solution(s) to lc problem(s): No.xxxx
• 修复错误：fix: xxxx
• 日常维护：chore: xxx

3. 其它补充

新增题解及代码时，需要创建 Solution.xxx 源代码文件（如果已存在，请确认算法是否更优，是则覆盖已有算法代码），同时，需要在 README.md 以及 README_EN.md 中添加对应的代码片段（英文文件中不要出现中文注释）

另外，编码风格（比如变量、函数的命名），尽量跟项目已有代码保持一致。

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🤭 Thank you for your contribution. Please check if your changes comply with the following project specifications.

1. Formatting

We use different formatting tools for various programming languages (including documentation) in our project. You must ensure that the code and documentation are correctly formatted before submitting a pr.

• .{md,js,ts,php,sql} use prettier
• .{c,cpp,java} use clang-format
• .{py} use black
• .{go} use gofmt
• Others to be improved

2. Git Commit Message

Our project follows the AngularJS Git Commit Message Conventions. We hope that your submission information is as consistent as possible with the project.

• Add or modify solutions: feat: add/update solution(s) to lc problem(s): No.xxxx
• Fix errors: fix: xxxx
• Routine maintenance: chore: xxx

3. Other notes

When adding solutions and code, you need to create a Solution.xxx source code file (if it already exists, please confirm whether the algorithm is better, if yes, overwrite the existing algorithm code), and at the same time, you need to add the corresponding code snippets in README.md and README_EN.md (do not have Chinese comments in the English file)

In addition, the coding style (such as the naming of variables and functions) should be as consistent as possible with the existing code in the project.

[yanglbme]

Looks good, thank you!