docs: swap example file storage away from deprecated lib

[stvstnfrd]

The S3BotoStorage backend was deprecated in favor of the S3Boto3Storage backend. This change updates the example backend to reference the latter.

This is meant to accompany openedx/edx-platform#32646

Fixes: FAL-3431
Fixes: public-engineering/128

Configuration Pull Request

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[stvstnfrd]

Note: This work is being done on behalf of OpenCraft and should, presumably, be covered under their CLA agreement.

CC: @openedx/cla-problems

[ChrisChV]

Looks good +1

• I read through the changes.

[stvstnfrd]

@openedx/cla-problems I've tried re-running the CLA check, but since the runner provides no visual feedback or indicator, I can't tell if it's actually working or not.

Related: Because the CLA check looks like it's been implemented with a webhook and not a Github Action, contributors can't manually rerun the check. If this were implemented with the latter, contributors could re-run the check explicitly, instead of relying on the "edit the title", which is a crude hack. Further, implementing this as a GHA would provide explicit, visual indication that the job was actually rerun (or not).

[pomegranited]

👍 -- this is ready to merge once openedx/edx-platform#32646 merges.

• I tested this -- actual change is on build: swap default file storage away from deprecated lib openedx/edx-platform#32646, which is being reviewed separately.
• I read through the code
• I checked for accessibility issues N/A
• Updates documentation

[stvstnfrd]

@pomegranited All green and ready to merge.