Documentation: Add a missing field to the extended config s3 example

[jontg]

What this PR does / why we need it:

Loki does not do a great job complaining when the S3 configuration is invalid - instead, apparently it tries to hang indefinitely. It then naturally follows that good documentation is required, so that we don't lose hours combing through code, force deleting pods, and the like.

This PR addresses a gap in documentation, where how to get a bucket name into the config is not demonstrated. Notably, the code suggests that this new parameter can be used to evenly distribute logs to multiple buckets, so we add that to the example.

Which issue(s) this PR fixes:
Fixes life-stealing frustrations and dental bills from gnashed teeth.

Special notes for your reviewer:

Checklist

• Documentation added
• Tests updated

[CLAassistant]

All committers have signed the CLA.

[codecov-io]

Codecov Report

Merging #2991 (de02424) into master (9ba5753) will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2991      +/-   ##
==========================================
+ Coverage   61.90%   61.96%   +0.06%     
==========================================
  Files         182      182              
  Lines       14890    14890              
==========================================
+ Hits         9218     9227       +9     
+ Misses       4826     4817       -9     
  Partials      846      846              

Impacted Files	Coverage Δ
pkg/querier/queryrange/limits.go	91.66% <0.00%> (-4.17%)	⬇️
pkg/logql/evaluator.go	91.64% <0.00%> (+0.40%)	⬆️
pkg/canary/comparator/comparator.go	78.18% <0.00%> (+1.81%)	⬆️
pkg/promtail/targets/file/filetarget.go	66.19% <0.00%> (+2.11%)	⬆️
pkg/querier/queryrange/downstreamer.go	97.64% <0.00%> (+2.35%)	⬆️

[cyriltovena]

LGTM