Fixes docker version number typo

[boonew2]

Issue #, if available: N/A

Description of changes:
Using docker 18 in the standard:4.0 image runtime hopefully no longer lies to people about being version 19.

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[subinataws]

@boonew2 - Thanks for the contribution. Note that you do not need to specify docker versions in the runtime anymore. CodeBuild will default select the latest version for you, once you set the privileged mode as true for the Project. If you do specify a Docker version, it will be ignored and the latest version will be picked for the latest CodeBuild images. The print statement is misleading so will accept the PR for local builds.

version: 0.2

phases:
  build:
    commands:
      - docker -v

[boonew2]

If you do specify a Docker version, it will be ignored and the latest version will be picked for the latest CodeBuild images

I noticed that the standard:3.0 runtime yml had a message that said that, but 4.0 went back to echo'ing the specific version so i wasn't sure what was happening behind the scenes.