Fix repetition in NonEmptyChain scala docs

[Obarros]

This will update the scaladocs for NonEmptyChain.
The same example was used for reduceRightTo and reduceLeftTo method.
Fix #2803

[codecov-io]

Codecov Report

Merging #2803 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2803   +/-   ##
=======================================
  Coverage   94.15%   94.15%           
=======================================
  Files         368      368           
  Lines        6914     6914           
  Branches      296      296           
=======================================
  Hits         6510     6510           
  Misses        404      404

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/NonEmptyChain.scala	98.14% <ø> (ø)	⬆️

---

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 4b562a8...3ec8426. Read the comment docs.

[LukaJCB]

This looks good to me, thanks Ozias :)

[kailuowang]

I was puzzled how the original example worked. Then I realized that it's the any2stringadd