Issue 4091: changed YML to YAML

[joewxboy]

Pull Request Template

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

Fixes #4091

Type of change

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• [X] Bug fix (non-breaking change which fixes an issue)

...

Checklist:

• [X] My code follows the style guidelines of this project
• [X] I have performed a self-review of my own code
• [X] I have commented my code, particularly in hard-to-understand areas
• [X] I have made corresponding changes to the documentation
• [X] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules
• [X] I have checked my code and corrected any misspellings
• [X] I have tagged the reviewers in a comment below incase my pull request is ready for a review
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@open-horizon/documentation-working-group

[joewxboy]

This will update the instructions on https://open-horizon.github.io/docs/installing/edge_cluster_agent/ so that we are kubectl apply-ing a file with the same extension as what we downloaded.