chore(codebuild): remove the reference in our docs to the old UBUNTU_…

…14_04_RUBY_2_5_1 image (aws#6682)

Closes aws#6517

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

packages/@aws-cdk/aws-codebuild/README.md

@@ -190,7 +190,7 @@ The CodeBuild library supports both Linux and Windows images via the
 
 You can either specify one of the predefined Windows/Linux images by using one
 of the constants such as `WindowsBuildImage.WIN_SERVER_CORE_2016_BASE` or
-`LinuxBuildImage.UBUNTU_14_04_RUBY_2_5_1`.
+`LinuxBuildImage.STANDARD_2_0`.
 
 Alternatively, you can specify a custom image using one of the static methods on
 `XxxBuildImage`: