Fix typo in Alternative docs

[SimY4]

Fixing a small typo in Alternative docs.

[codecov-io]

Codecov Report

Merging #3173 into master will decrease coverage by 2.76%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3173      +/-   ##
==========================================
- Coverage   93.37%   90.61%   -2.77%     
==========================================
  Files         372      375       +3     
  Lines        7188     7352     +164     
  Branches      194      202       +8     
==========================================
- Hits         6712     6662      -50     
- Misses        476      690     +214

Flag	Coverage Δ
#scala_version_212	?
#scala_version_213	90.61% <ø> (?)

Impacted Files	Coverage Δ
core/src/main/scala/cats/syntax/ior.scala	0% <0%> (-100%)	⬇️
core/src/main/scala/cats/syntax/nested.scala	0% <0%> (-100%)	⬇️
...re/src/main/scala/cats/syntax/traverseFilter.scala	0% <0%> (-100%)	⬇️
core/src/main/scala/cats/syntax/set.scala	0% <0%> (-100%)	⬇️
core/src/main/scala/cats/syntax/TrySyntax.scala	0% <0%> (-100%)	⬇️
core/src/main/scala/cats/syntax/function1.scala	0% <0%> (-100%)	⬇️
core/src/main/scala/cats/syntax/eitherK.scala	0% <0%> (-100%)	⬇️
core/src/main/scala/cats/syntax/alternative.scala	28.57% <0%> (-71.43%)	⬇️
core/src/main/scala/cats/syntax/option.scala	30% <0%> (-70%)	⬇️
core/src/main/scala/cats/syntax/validated.scala	40% <0%> (-60%)	⬇️
... and 53 more

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[travisbrown]

Thanks!