docs: update android buildTypes example in flavors guide

[lamek]

Updated the build.gradle.kts snippet to reflect that the default Flutter template only explicitly configures the release build type, leaving debug as implicit

#12205

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[gemini-code-assist]

Code Review

This pull request updates the documentation in flavors.md regarding the configuration of build types in build.gradle.kts for Flutter. The review feedback correctly points out that using release { ... } directly in Gradle Kotlin DSL can cause compilation errors, and suggests using getByName("release") instead to ensure compatibility and match the default Flutter template.

[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit 94827a0):

https://flutter-docs-prod--docs-pr13674-fix-12205-655f5goj.web.app

[flutter-website-bot]

Staged preview of the updated flutter.dev site (updated for commit 94827a0):

https://flutter-dev-230821--www-pr13674-fix-12205-n2m5wrqe.web.app