Set multipart_chunk_size_mb in s3cfg backups storage examples

What does this MR do?

Set multipart_chunk_size_mb in s3cfg backups storage examples

The value multipart_chunk_size_mb = 128 is selected to match the minio configuration:
https://gitlab.com/gitlab-org/charts/gitlab/-/blob/397da18fe99d870b77492c981d49384427473e70/charts/gitlab/charts/task-runner/templates/configmap.yaml#L127

Otherwise, the s3cmd default value of 15MB is used and large backups have the following error:

ERROR: Parameter problem: Chunk size 15 MB results in more than 10000 chunks. Please increase --multipart-chunk-size-mb

Related issues

Minio configuration: #1495 (closed) !884 (merged)

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(Only documentation changed)