chore(manifests): Fix broken github link

[Tomcli]

Which issue is resolved by this Pull Request:
Resolves #

Description of your changes:
Tekton Dashboard yamls are no longer hosted on Github. Instead they all hosted on GCS.

Environment tested:

• Python Version (use python --version):
• Tekton Version (use tkn version):
• Kubernetes Version (use kubectl version):
• OS (e.g. from /etc/os-release):

Checklist:

• The title for your pull request (PR) should follow our title convention. Learn more about the pull request title convention used in this repository.

[Tomcli]

/assign @yhwang

[yhwang]

/lgtm

[google-oss-prow]

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[yhwang]

@Tomcli don't we have the readme link checker?

[Tomcli]

@Tomcli don't we have the readme link checker?

The readme link checker doesn't check embedded links in code block. Also some code block just have placeholder links so we probably need to make the link checker opt out certain files if we want to update it to also check for code block links.