Jsonet: Fixes typo in shipper.libjsonnet

[riwar]

What this PR does / why we need it:

Changes line 43 in production/ksonnet/loki shipper.json from:

shared_store: if self.using_tsdb_shipper then self.tsdb_shipper_shared_store else self.boltdb_shipper_shared_store,

to

shared_store: if $._config.using_tsdb_shipper then $._config.tsdb_shipper_shared_store else $._config.boltdb_shipper_shared_store,

In order to fix "field using_tsdb_shipper not found"

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• CHANGELOG.md updated
  • If the change is worth mentioning in the release notes, add add-to-release-notes label
• Changes that require user attention or interaction to upgrade are documented in docs/sources/upgrading/_index.md
• For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR

[CLAassistant]

All committers have signed the CLA.

[vlad-diachenko]

LGTM, thanks @riwar