[OKTA-1262776] Add act claim to token exchange guide

[thomascavanagh-okta]

Description:

• What's changed? Adding the act claim to example snippets and brief explainers.
• Is this PR related to a Monolith release? Yes, TBD

Resolves:

• OKTA-1262776

Vercel Preview Link:

[okta-prod-github-app]

Acrolinx score

A minimum Acrolinx Score of 80 is required. The total score is an average of the subscores.
Select Total score to review the Acrolinx scorecard for your article. Try to increase your individual scores, for example: Correctness. Your content will be clearer and more consistent.

Article	Total score Required:80	Word and phrases (Brand, terms) Preferred: 80	Correctness (Spelling, grammar) Preferred: 80	Clarity (Readability) Preferred: 80	Inclusive language (+ accesibility) Preferred: 80
✅ packages/@okta/vuepress-site/docs/guides/set-up-token-exchange/main/index.md	85	100	83	72

Successfully checked 1 of 1 documents.
See summary in Content Analysis Dashboard

Reopen the pull request or push new changes to check again.

Depending on the Acrolinx server configuration, the
links expire after some time and you must have a login for the
Acrolinx server to access them again.