Fix argument name in `cfr_time_varying()` documentation

[joshwlambert]

This PR fixes a possible typo in the documentation of cfr_time_varying() which references what is maybe a previous version of the delay_density argument, epidist.

• [x] I have read the CONTRIBUTING guidelines
• [ ] A new item has been added to NEWS.md
• [ ] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been added / updated (for bug fixes / features)
• [x] Checks have been run locally and pass

This PR doesn't not contain any (functional) user-facing or breaking changes.

[pratikunterwegs]

Just nudging @joshwlambert on this.

[joshwlambert]

Thanks for the nudge @pratikunterwegs (or should I say Richard Thaler).

I've made a small change to the doc as mentioned in the message above, and now all the checks pass so I'll merge this PR.

[pratikunterwegs]

Thanks @joshwlambert! Although I will point out that behavioural economics is somewhat on the ropes these days :)