multi-tls readme example to reference the file

[maximbr]

Makes the doc on https://kubernetes.github.io/ingress-nginx/examples/multi-tls/ clearer by linking to the example file (no directory context there as in Github so the example is not clear)

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: maximbr
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[maximbr]

CLA check

[aledbf]

/lgtm
/approve

[aledbf]

@maximbr thanks!