Fix README links to use main branch (tailwindlabs#19641)

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## Summary
The README still referenced the `next` branch for the CI badge and
contributing docs link, but the `next` branch no longer exists upstream.
This updates those URLs to point to `main` so the badge and contributing
docs link work correctly.
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Provide a summary of the issue and the changes you're making. How does
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## Test plan
N/A, docs only change 
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Author: pavan-sh

README.md

@@ -13,7 +13,7 @@
 </p>
 
 <p align="center">
-    <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://github.com/tailwindlabs/tailwindcss/actions"><img src="https://img.shields.io/github/actions/workflow/status/tailwindlabs/tailwindcss/ci.yml?branch=main" 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/tailwindlabs/tailwindcss/blob/main/LICENSE"><img src="https://img.shields.io/npm/l/tailwindcss.svg" alt="License"></a>
@@ -33,4 +33,4 @@ For help, discussion about best practices, or feature ideas:
 
 ## Contributing
 
-If you're interested in contributing to Tailwind CSS, please read our [contributing docs](https://github.com/tailwindcss/tailwindcss/blob/next/.github/CONTRIBUTING.md) **before submitting a pull request**.
+If you're interested in contributing to Tailwind CSS, please read our [contributing docs](https://github.com/tailwindcss/tailwindcss/blob/main/.github/CONTRIBUTING.md) **before submitting a pull request**.