add missing link to diagnose method in CmdStanModel doc

[jgabry]

Submission Checklist

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

Summary

closes #829

Adds missing link to diagnose method in CmdStanModel doc

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):
Columbia University

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 #833 (7af3663) into master (a9c898b) will decrease coverage by 1.70%.
The diff coverage is n/a.

❗ Current head 7af3663 differs from pull request most recent head 2b096f5. Consider uploading reports for the commit 2b096f5 to get more accurate results

@@            Coverage Diff             @@
##           master     #833      +/-   ##
==========================================
- Coverage   88.30%   86.60%   -1.70%     
==========================================
  Files          12       12              
  Lines        4182     4182              
==========================================
- Hits         3693     3622      -71     
- Misses        489      560      +71     

Files Changed	Coverage Δ
R/model.R	90.48% <ø> (-0.14%)	⬇️

... and 4 files with indirect coverage changes

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