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

[bestickley]

I was confused in the docs by this:

const pipeline = new GitHubWorkflow(app, 'Pipeline', {
  synth: new ShellStep('Build', {
    commands: [
      'yarn install',
      'yarn build',
    ],
  }),
  awsCreds: AwsCredentials.fromOpenIdConnect({
    gitHubActionRoleArn: 'arn:aws:iam::<account-id>: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 have.