docs: update deploy command

[badmintoncryer]

The README states the deployment command as npx cdk deploy, but in reality, the --all option is required.

$ npx cdk deploy      
Bundling asset ServerlessWebappStarterKitUsEast1Stack/SignPayloadHandler/Handler/Code/Stage...

  ...84148bb8321ddc842250d6cdb82d2fb89f14d4ac4130f7bd5b3a064d3-building/index.js  1.5kb

⚡ Done in 26ms
[WARNING] aws-cdk-lib.aws_ec2.InstanceProps#keyName is deprecated.
  - Use `keyPair` instead - https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_ec2-readme.html#using-an-existing-ec2-key-pair
  This API will be removed in the next major release.
Since this app includes more than a single stack, specify which stacks to use (wildcards are supported) or specify `--all`
Stacks: ServerlessWebappStarterKitUsEast1Stack · ServerlessWebappStarterKitStack

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[tmokmss]

Thanks!!