fix optimize doc Jacobian description

[avehtari]

@fweber144 pointed out that cmdstanr::optimize doc did not match CmdStan guide. This PR aligns the docs.

Copyright and Licensing

Aki Vehtari

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 87.15%. Comparing base (d7d3999) to head (c59b5b9).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1115   +/-   ##
=======================================
  Coverage   87.15%   87.15%           
=======================================
  Files          14       14           
  Lines        5972     5973    +1     
=======================================
+ Hits         5205     5206    +1     
  Misses        767      767           

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