update code-hints in engine reference

Regenerated the yaml files with the latest version from the
19.03 branch on docker/cli

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

_data/engine-cli/docker_build.yaml

@@ -717,7 +717,7 @@ examples: |-
   specify an intermediate build stage by name as a final stage for the resulting
   image. Commands after the target stage will be skipped.
 
-  ```Dockerfile
+  ```dockerfile
   FROM debian AS build-env
   ...
 
@@ -785,7 +785,7 @@ examples: |-
   The example `Dockerfile` below uses a separate stage to collect the
   build-artifacts for exporting:
 
-  ```Dockerfile
+  ```dockerfile
   FROM golang AS build-stage
   RUN go get -u github.com/LK4D4/vndr
 
@@ -948,7 +948,7 @@ examples: |-
 
   The following is an example of docker build with `--squash` argument
 
-  ```Dockerfile
+  ```dockerfile
   FROM busybox
   RUN echo hello > /hello
   RUN echo world >> /hello

_data/engine-cli/docker_pull.yaml

@@ -179,7 +179,7 @@ examples: |-
 
   Digest can also be used in the `FROM` of a Dockerfile, for example:
 
-  ```Dockerfile
+  ```dockerfile
   FROM ubuntu@sha256:45b23dee08af5e43a7fea6c4cf9c25ccf269ee113168c19722f87876677c5cb2
   MAINTAINER some maintainer <maintainer@example.com>
   ```