Fix FunctorFilter mapFilter doc example

[regadas]

Thank you for contributing to Cats!

This is a kind reminder to run sbt prePR and commit the changed files, if any, before submitting.

[LukaJCB]

Thank you @regadas :)

[codecov-io]

Codecov Report

Merging #2967 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2967      +/-   ##
==========================================
- Coverage   93.93%   93.91%   -0.03%     
==========================================
  Files         371      371              
  Lines        7061     7061              
  Branches      177      177              
==========================================
- Hits         6633     6631       -2     
- Misses        428      430       +2

Impacted Files	Coverage Δ
core/src/main/scala/cats/FunctorFilter.scala	100% <ø> (ø)	⬆️
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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[codecov-io]

Codecov Report

Merging #2967 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2967      +/-   ##
==========================================
- Coverage   93.93%   93.91%   -0.03%     
==========================================
  Files         371      371              
  Lines        7061     7061              
  Branches      177      177              
==========================================
- Hits         6633     6631       -2     
- Misses        428      430       +2

Impacted Files	Coverage Δ
core/src/main/scala/cats/FunctorFilter.scala	100% <ø> (ø)	⬆️
testkit/src/main/scala/cats/tests/Helpers.scala	96% <0%> (-4%)	⬇️
...cala/cats/kernel/instances/FunctionInstances.scala	96.96% <0%> (-3.04%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0bab9eb...afc2c7b. Read the comment docs.