Fix typo in ODE algorithms: Use FBDF for problems with > 50 states

[devmotion]

There are two identical conditions length(prob.u0) > 500 in a single if - elseif - ... statement, so the second one is never hit. Based on the branches for problems with :auto hints below and the logic prior to #977, I assume that the second condition was introduced accidentally in #977 and was supposed to be length(prob.u0) > 50.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.43%. Comparing base (8ce23d2) to head (b097bfd).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1024   +/-   ##
=======================================
  Coverage   85.43%   85.43%           
=======================================
  Files          11       11           
  Lines         151      151           
=======================================
  Hits          129      129           
  Misses         22       22           

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