[documentation] [url_launcher] fix for readme code sample

[iapicca]

Description

the code sample at this link and reported below is clearly broken

import 'package:flutter/material.dart';
import 'package:url_launcher/url_launcher.dart';

void main() {
  runApp(Scaffold(
    body: Center(
      child: RaisedButton(
        onPressed: _launchURL,
        child: Text('Show Flutter homepage'),
      ),
    ),
  ));
}

_launchURL() async {
  const url = 'https://flutter.dev';
  if (await canLaunch(url)) {
    await launch(url);
  } else {
    throw 'Could not launch $url';
  }
}

please consider replacing it with the following

import 'package:flutter/material.dart';
import 'package:url_launcher/url_launcher.dart';

const _url = 'https://flutter.dev';

void main() => runApp(
      const MaterialApp(
        home: Material(
          child: Center(
            child: RaisedButton(
              onPressed: _launchURL,
              child: Text('Show Flutter homepage'),
            ),
          ),
        ),
      ),
    );

void _launchURL() async =>
    await canLaunch(_url) ? await launch(_url) : throw 'Could not launch $_url';

Related Issues

flutter/flutter#71923

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process.

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See [Contributor Guide]).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the [Flutter Style Guide].
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I updated pubspec.yaml with an appropriate new version according to the [pub versioning philosophy].
• I updated CHANGELOG.md to add a description of the change.
• I signed the [CLA].
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

[hamdikahloun]

Please update CHANGELOG.md & pubspec.yaml

[iapicca]

@hamdikahloun

Please update CHANGELOG.md & pubspec.yaml

bumped version: 5.7.12 in pubspec.yaml

and updated CHANGELOG.md

## 5.7.12

* Updated code sample in `README.md`

[hamdikahloun]

LGTM

[hamdikahloun]

Can you merge this up to head to re-run CI in current state?