Fixing typo in non-centered double exponential

[jsocolar]

Submission Checklist

• Builds locally
• New functions marked with `r since("VERSION")`
• Declare copyright holder and open-source license: see below

Summary

Fixing a typo. See https://discourse.mc-stan.org/t/mis-understanding-reparameterization-of-the-double-exponential-distribution/32785/4

No new functions. I'm having trouble building locally but it's just a series of bookdown/pandoc/tex issues.

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company):

Jacob B. Socolar

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC BY-ND 4.0 (https://creativecommons.org/licenses/by-nd/4.0/)

[jgabry]

Thanks Jacob!