docs: update Dart SDK version constraints (#1059)

<!--
  Thanks for contributing!

Provide a description of your changes below and a general summary in the
title

Please look at the following checklist to ensure that your PR can be
accepted quickly:
-->

## Description

It seems that the example `pubspec.yaml` Dart SDK version constraints
got bumped but the documentation did not.


https://github.com/flutter/news_toolkit/blob/553992b00cccd0d2e51fa9b1f71803980c1b637b/flutter_news_example/pubspec.yaml#L6

## Type of Change

<!--- Put an `x` in all the boxes that apply: -->

- [ ] ✨ New feature (non-breaking change which adds functionality)
- [ ] 🛠️ Bug fix (non-breaking change which fixes an issue)
- [ ] ❌ Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] 🧹 Code refactor
- [ ] ✅ Build configuration change
- [X] 📝 Documentation
- [ ] 🗑️ Chore

docs/docs/overview.md

@@ -32,7 +32,7 @@ Depending on the number of flavors you plan to create for your project, the setu
 In order to generate a project using the news template, you must have the [Dart SDK][dart_installation_link] installed on your machine.
 
 :::info
-Dart `">=2.18.0 <3.0.0"` is required.
+Dart `">=3.0.0 <4.0.0"` is required.
 :::
 
 **Mason**