Fix README.md

[jon-rom]

Fix README to reference BigQuery

resolves #
docs dbt-labs/docs.getdbt.com/#

Problem

README file reference a not BigQuery datawarehouse

Solution

Fix to BigQuery.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• [] This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX