📖 fix a link for Cluster API videos and tutorials

[pacoxu]

What this PR does / why we need it:
/kind documentation

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes None

Wrong link in https://cluster-api.sigs.k8s.io/developer/providers/getting-started/overview#we-need-your-help.

• Find [Cluster API videos and tutorials](https://cluster-api.sigs.k8s.io/developer/guide.html#videos-explaining-capi-architecture-and-code-walkthroughs).

[k8s-ci-robot]

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[pacoxu]

/area documentation

[sbueringer]

Thx!

/lgtm
/approved

/cherry-pick release-1.9

[k8s-infra-cherrypick-robot]

@sbueringer: once the present PR merges, I will cherry-pick it on top of release-1.9 in a new PR and assign it to you.

In response to this:

Thx!

/lgtm
/approved

/cherry-pick release-1.9

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[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 6fa6c691ed244f82401bba31ef870cbc7bcf3adf