Replace latest badge with dev badge

[hyrodium]

https://juliadiff.org/DiffRules.jl/latest is not up-to-date. This PR replaces the link with https://juliadiff.org/DiffRules.jl/dev.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (8842177) 97.88% compared to head (a2c2769) 97.88%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #103   +/-   ##
=======================================
  Coverage   97.88%   97.88%           
=======================================
  Files           3        3           
  Lines         189      189           
=======================================
  Hits          185      185           
  Misses          4        4           

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