Improve docs block extraction in update-relation-profile.sh using `awk`

[stumelius]

Description & motivation

Checklist

• [ ] I have verified that these changes work locally on the following warehouses (Note: it's okay if you do not have access to all warehouses, this helps us understand what has been covered):
  • [ ] Postgres
  • [ ] BigQuery
  • [ ] Snowflake
  • [ ] Redshift
  • [ ] SQL Server
  • [ ] Databricks
• [ ] I have written tests for new macros (either as dbt schema tests in integration_tests/models, dbt data tests in integration_tests/tests or integration tests in the CI workflow)
• [ ] I have updated the README.md (if applicable)