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

Merging #686 (4e1af05) into master (9ad2918) will decrease coverage by 2.24%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #686      +/-   ##
==========================================
- Coverage   89.95%   87.71%   -2.25%     
==========================================
  Files          12       12              
  Lines        3654     3654              
==========================================
- Hits         3287     3205      -82     
- Misses        367      449      +82     

Impacted Files	Coverage Δ
R/install.R	49.41% <0.00%> (-13.52%)	⬇️
R/csv.R	93.84% <0.00%> (-3.83%)	⬇️
R/utils.R	78.90% <0.00%> (-0.85%)	⬇️
R/run.R	94.79% <0.00%> (-0.48%)	⬇️
R/model.R	91.14% <0.00%> (-0.15%)	⬇️

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[rok-cesnovar]

Thanks!