Fix incorrect adapter names in Dockerfile READMEs

[sunilp]

Summary

The Docker READMEs for dbt-bigquery, dbt-redshift, and dbt-spark incorrectly reference "Snowflake" in example descriptions, even though the build targets and commands correctly reference their respective adapters.

This PR replaces "Snowflake" with the correct adapter name in each file:

• dbt-bigquery/docker/README.md → "BigQuery"
• dbt-redshift/docker/README.md → "Redshift"
• dbt-spark/docker/README.md → "Spark"

Fixes #1700

Checklist

• I have signed the CLA
• This PR fixes a documentation bug (no code changes)

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[sunilp]

Thanks for your pull request, and welcome to our community! We require contributors to sign our Contributor License Agreement and we don't seem to have your signature on file. Check out this article for more information on why we have a CLA.

In order for us to review and merge your code, please submit the Individual Contributor License Agreement form attached above above. If you have questions about the CLA, or if you believe you've received this message in error, please reach out through a comment on this PR.

CLA has not been signed by users: @sunilp

already have signed

[sunilp]

Friendly bump — CLA is signed and verified (status check passing). The CI check-run-approval step looks like it needs a maintainer to approve the workflow run for first-time contributors. Would appreciate a review when you get a chance!