Fix outdated syntax

[JPrevost]

Redis was misconfigured for a long time in timdex-api. That was patched yesterday. On doing so, it exposed this incorrect configuration in rack-attack that was missed during a version upgrade.

https://mitlibraries.atlassian.net/browse/TIMX-679

This was already fixed in TIMDEX-UI and only missed in TIMDEX API because rack-attack was not realized to have been not working for different reasons.

Developer

• All new ENV is documented in README
• All new ENV has been added to Heroku Pipeline, Staging and Prod
• ANDI or Wave has been run in accordance to
  our guide and
  all issues introduced by these changes have been resolved or opened as new
  issues (link to those issues in the Pull Request details above)
• Stakeholder approval has been confirmed (or is not needed)

Code Reviewer

• The commit message is clear and follows our guidelines
  (not just this pull request message)
• There are appropriate tests covering any new functionality
• The documentation has been updated or is unnecessary
• The changes have been verified
• New dependencies are appropriate or there were no changes

Requires database migrations?

NO

Includes new or updated dependencies?

NO