[release-15.0] Fix release script for the version in the docker script

[GuptaManan100]

Description

This PR fixes the release script for updating the correct version in the docker script for release-15.0.

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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