Update broken link for Stan OpenCL support page

[erictleung]

Submission Checklist

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

Summary

Fixes broken OpenCL support link, which appears to be this instead http://mc-stan.org/math/d0/d8f/opencl_support.html

Closes #685

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):
Eric Leung

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/)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.95%. Comparing base (9ad2918) to head (4e1af05).
⚠️ Report is 832 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #686   +/-   ##
=======================================
  Coverage   89.95%   89.95%           
=======================================
  Files          12       12           
  Lines        3654     3654           
=======================================
  Hits         3287     3287           
  Misses        367      367           

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[rok-cesnovar]

Thanks!