Fix Foldable doctest to use separateFoldable instead of separate

[gagandeepkalra]

Minor though, found this when I was working on backporting separateFoldable to scala_2.11

[codecov-io]

Codecov Report

Merging #3342 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3342   +/-   ##
=======================================
  Coverage   93.31%   93.31%           
=======================================
  Files         378      378           
  Lines        7689     7689           
  Branches      206      206           
=======================================
  Hits         7175     7175           
  Misses        514      514

Flag	Coverage Δ
#scala_version_212	93.38% <ø> (ø)	⬆️
#scala_version_213	93.09% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
core/src/main/scala/cats/Alternative.scala	92.3% <ø> (ø)	⬆️

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[travisbrown]

👍, good catch, thanks.