Fix broken URL in README

[TennyZhuang]

No description provided.

[CLAassistant]

All committers have signed the CLA.

[tisonkun]

@TennyZhuang please sign CLA and we can proceed.

[TennyZhuang]

@TennyZhuang please sign CLA and we can proceed.

Done

[tisonkun]

@rebelice do you have an idea why "Build & Test" doesn't run?

[tisonkun]

It seems the workflow configures wrongly. Let me try to fix it.

[tisonkun]

It is too complex to a course IMO. The Makefile is quite outdated and also the code. I don't want to fix them anyway.

cc @zhangyangyu @rebelice you may take a look at this issue. The build system of tinysql is quite mess.

[rebelice]

It's too hard to fix Build & Test. As far as I know, CI will keep failing until the entire tinysql is completed.

[tisonkun]

@rebelice I agree. Any progress on #83 ?

[rebelice]

@tisonkun Yes, I'm working on pushing the first PR. You may see it later.

[tisonkun]

@TennyZhuang thanks for your contribution. Although I think the contribution guide should be updated later.

[tisonkun]

Contribution guides of tinysql & tidb are different also.