Improve Snowplow docs SQL reference

What does this MR do?

The SQL reference in the Snowplow docs is missing the --important-- session_id field and has an extra one (v_tracker). This small MR addresses this.

Does this MR meet the acceptance criteria?

Conformity

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  • I have included a changelog entry.
  • I have not included a changelog entry because this is a docs-only change.
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