Clarify tagging inheritance example

[deanna-minnick]

Description & motivation

This PR improves naming consistency and completes the logical flow in the source tag example at the bottom of tags.md. I confirmed that this logic works as expected in my own dbt project using the code below and running each permutation of dbt test -s tag:<tag_level> (ie dbt test -s tag:top_level).

version: 2

sources:
  - name: jaffle_shop
    tags: ['top_level']
    database: raw
    schema: jaffle_shop
    tables:
      - name: customers
        tags: ['table_level']
        columns:
          - name: id
            tags: ['column_level']
            tests:
              - unique:
                  tags: ['test_level']
      - name: orders```

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[mirnawong1]

thanks for adding that clarification @deanna-minnick !