CircleCI docs: fix invalid executor key (#2607)

executorType is no longer a valid executor definition. The simplest form to designate using machine executor is `machine: true`

docs/supported_docker_environment/continuous_integration/circle_ci.md

@@ -9,7 +9,7 @@ Here is a sample CircleCI configuration that does a checkout of a project and ru
 # Check https://circleci.com/docs/2.0/language-java/ for more details
 #
 version: 2
-executorType: machine
+machine: true
 jobs:
   build:
     steps: