[DOC] update path to requirements for readthedocs

[minrk]

What does this PR do?

RTD builds haven't succeeded since docs/requirements.txt was moved in #748, so the latest docs build is for 2.4.0.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.29%. Comparing base (cb2eb37) to head (44b9e06). Report is 13 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #805 +/- ## ========================================== - Coverage 91.54% 91.29% -0.25% ========================================== Files 17 17 Lines 1621 1632 +11 ========================================== + Hits 1484 1490 +6 - Misses 137 142 +5 ```

[willingc]

Thanks @minrk! I hope all is well.