DOCS-647 - Add target fields to threat intel article

[jpipkin1]

Purpose of this pull request

This pull request updates this article:
https://help.sumologic.com/docs/cse/administration/create-custom-threat-intel-source/#target-fields-for-threat-indicators

Select the type of change

• Minor Changes - Typos, formatting, slight revisions
• Update Content - Revisions, updating sections
• New Content - New features, sections, pages, tutorials
• Site and Tools - .clabot, version updates, maintenance, dependencies, new packages for the site (Docusaurus, Gatsby, React, etc.)

Ticket (if applicable)

DOCS-647