docs: `yarn build` -> `yarn synth`

I was confused in the docs by this:

```ts
const pipeline = new GitHubWorkflow(app, 'Pipeline', {
synth: new ShellStep('Build', {
commands: [
'yarn install',
'yarn build',
],
}),
awsCreds: AwsCredentials.fromOpenIdConnect({
gitHubActionRoleArn: 'arn:aws:iam:::role/GitHubActionRole',
}),
});
```

because I didn't see where the synth command was being run. I think it would be more clear if you changed `yarn build` to `yarn synth` like the [CDK Pipeline docs](https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.pipelines-readme.html) have.