Fix dead link in docs

[BioTurboNick]

Pointed to a README file in dev that doesn't exist.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.94%. Comparing base (eb6492c) to head (abc5fc7).

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #2403       +/-   ##
===========================================
+ Coverage   43.04%   73.94%   +30.89%     
===========================================
  Files          32       32               
  Lines        1856     1911       +55     
===========================================
+ Hits          799     1413      +614     
+ Misses       1057      498      -559     

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[mcabbott]

Thanks.

Sadly this needs updating, opt.eta = next!(schedule) will only work for old-style implicit optimisers... needs to become something like Flux.adjust!(opt_state, eta = next!(schedule)) for new-style ones.