[DOC] Fix optionT link in monad documentation

[CucumisSativus]

Small, but maybe it will make someone's day better :)

[codecov-io]

Codecov Report

Merging #3020 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3020      +/-   ##
==========================================
- Coverage   93.56%   93.53%   -0.03%     
==========================================
  Files         368      368              
  Lines        6949     6949              
  Branches      195      195              
==========================================
- Hits         6502     6500       -2     
- Misses        447      449       +2

Impacted Files	Coverage Δ
testkit/src/main/scala/cats/tests/Helpers.scala	96% <0%> (-4%)	⬇️
...cala/cats/kernel/instances/FunctionInstances.scala	96.96% <0%> (-3.04%)	⬇️

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