fix link in readme

[shawn0915]

What problem does this PR solve?

Issue Number: close #xxx

Problem Summary:

What is changed and how it works:

fix link in readme, and remove $ so that everyone can copy/use command directly.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Notable changes

• Has configuration change
• Has HTTP API interfaces change
• Has tiproxyctl change
• Other user behavior changes

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

Welcome @shawn0915! It looks like this is your first PR to pingcap/TiProxy 🎉

[ti-chi-bot]

Hi @shawn0915. Thanks for your PR.

I'm waiting for a pingcap member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xhebox

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• OWNERS [xhebox]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-08-23 04:11:01.944127675 +0000 UTC m=+1295426.493143663: ☑️ agreed by xhebox.
• 2023-08-23 04:11:26.39165355 +0000 UTC m=+1295450.940669537: ✖️🔁 reset by xhebox.
• 2023-08-23 04:12:06.455114496 +0000 UTC m=+1295491.004130503: ☑️ agreed by xhebox.