Fix typo in show.md

[ollyw]

Small doc change to add a missing space, which may misleads users into thinking there is a library called kittensa instead of kittens:

From

Thankfully with the help of a small library called kittensa lot of type class instances including Show can be derived automatically!

To

Thankfully with the help of a small library called kittens a lot of type class instances including Show can be derived automatically!

[codecov-io]

Codecov Report

Merging #2921 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2921   +/-   ##
=======================================
  Coverage   94.24%   94.24%           
=======================================
  Files         367      367           
  Lines        7002     7002           
  Branches      195      195           
=======================================
  Hits         6599     6599           
  Misses        403      403

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