Tidy reference doc generation guide (#14825)

- fix YAML marked as shell
- trim trailing whitespace

content/en/docs/contribute/generate-ref-docs/kubernetes-components.md

@@ -97,7 +97,7 @@ Open `<web-base>/update-imported-docs/reference.yml` for editing.
 Do not change the content for the `generate-command` entry unless you understand
 what it is doing and need to change the specified release branch.
 
-```shell
+```yaml
 repos:
 - name: reference-docs
   remote: https://github.com/kubernetes-incubator/reference-docs.git
@@ -138,7 +138,7 @@ to the same destination directory, you can use wildcards in the value given to
 `src` and you can just provide the directory name as the value for `dst`.
 For example:
 
-```shell
+```yaml
   files:
   - src: gen-compdocs/build/kubeadm*.md
     dst: content/en/docs/reference/setup-tools/kubeadm/generated/