Update broken link in opencl.Rmd

[eipi10]

Current link is broken. I've changed it to what appears to be the current correct link, but I'm not positive.

Submission Checklist

• Run unit tests
• [ x] Declare copyright holder and agree to license (see below)

Summary

Please describe the purpose of the pull request.

Purpose: Fix broken link in OpenCL Vignette.

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):
Joel Schwartz

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 4.0 (https://creativecommons.org/licenses/by/4.0/)

[jgabry]

Thanks! That looks like the right link. I’ll let the unit tests run before merging even though this shouldn’t affect anything.

[jgabry]

One test is failing that has nothing to do with this PR so I'm going to go ahead and merge this. We'll have to look into that test separately (it seems to have something to do with CmdStan's tol_rel_grad option when using optimize. Perhaps something changed in the recent release, not sure at the moment.)