Fix README license badge link (tailwindlabs#19638)

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## Summary
The README license badge linked to a non-existent `master` branch (and
the wrong org).
This updates the link to the current location:
tailwindlabs/tailwindcss/blob/main/LICENSE.
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## Test plan
- Opened `README.md` and clicked the license badge/link to confirm it
resolves to the LICENSE file on the `main` branch.
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Author: pavan-sh

README.md

@@ -16,7 +16,7 @@
     <a href="https://github.com/tailwindlabs/tailwindcss/actions"><img src="https://img.shields.io/github/actions/workflow/status/tailwindlabs/tailwindcss/ci.yml?branch=next" alt="Build Status"></a>
     <a href="https://www.npmjs.com/package/tailwindcss"><img src="https://img.shields.io/npm/dt/tailwindcss.svg" alt="Total Downloads"></a>
     <a href="https://github.com/tailwindcss/tailwindcss/releases"><img src="https://img.shields.io/npm/v/tailwindcss.svg" alt="Latest Release"></a>
-    <a href="https://github.com/tailwindcss/tailwindcss/blob/master/LICENSE"><img src="https://img.shields.io/npm/l/tailwindcss.svg" alt="License"></a>
+    <a href="https://github.com/tailwindlabs/tailwindcss/blob/main/LICENSE"><img src="https://img.shields.io/npm/l/tailwindcss.svg" alt="License"></a>
 </p>
 
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