Update git-clone task instructions per 0.8

[concaf]

Changes

Recently, the git-clone task was updated to version 0.8 which is not backwards compatible in that it is run as a non-root user which does not have access to volumes mounted via workspaces by default. To accomodate this change, the users need to update their PipelineRuns and TaskRuns with podTemplate.securityContext.fsGroup = 65532 to fix the permissions on mounted volumes.

This commit makes this configuration change in the how-to guide.

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• [x] Includes docs (if user facing)
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[bendory]

/lgtm

[tekton-robot]

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In response to [this](https://github.com/tektoncd/website/pull/425#issuecomment-1219614504): >/lgtm Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.

[tekton-robot]

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[geriom]

/lgtm