Case Study: rv-multi.ipynb MCMC sampler RuntimeError: Chain 0 failed

**Describe the bug**
Case Study: rv-multi.ipynb MCMC sampler RuntimeError: Chain 0 failed

**To Reproduce**
Python 3.8.5
exoplanet.__version__ = '0.5.1'
Run case study rv-multi.ipynb
at https://gallery.exoplanet.codes/tutorials/rv-multi/
**Expected behavior**
Sampling 2 chains for 1_000 tune and 1_000 draw iterations (2_000 + 2_000 draws total) took 51 seconds.

**Your setup (please complete the following information):**
- Version of exoplanet: exoplanet.__version__ = '0.5.1'
- Operating system: MacOS Big sur 11.3
- Python version & installation method (pip, conda, etc.): Python 3.8.5 conda environment

**Additional context**
Add any other context about the problem here.
See attached screen print

Start with the rv-multi.ipynb case study at the linked tutorial, using Python 3.8.5, exoplanet 0.5.1, and the reported macOS setup. Reproduce the RuntimeError: Chain 0 failed and compare the result with the expected two-chain sampling output; the issue is resolved when the case study runs successfully.

Written by the indexing model from the issue text.