COMP: yaml language annotation on module_workflows.md

Addresses:

> /home/matt/src/ITK/Documentation/docs/contributing/module_workflows.md:20: WARNING: Pygments lexer name 'yml' is not known

If yml is used. Provide language markup.

Documentation/docs/contributing/module_workflows.md

@@ -17,7 +17,7 @@ ITK reusable workflows support most ITK external modules and provide CI boilerpl
 
 It is easy to add ITK reusable workflow integration to an external module. In a file `workflow.yml` in the `.github/workflows` directory of your project, add the following specification:
 
-```
+```yaml
 name: Build, test, package
 
 on: [push,pull_request]