Fix key name

[ghost]

dib -> div. That's what the Decoder uses :)

[codecov-io]

Codecov Report

Merging #7 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master       #7   +/-   ##
=======================================
  Coverage   54.54%   54.54%           
=======================================
  Files           1        1           
  Lines          66       66           
=======================================
  Hits           36       36           
  Misses         30       30

Impacted Files	Coverage Δ
src/main/scala/io/circe/spire/SpireCodecs.scala	54.54% <0%> (ø)	⬆️

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[LukaJCB]

Thanks!
Hopefully this will be better once we fix typelevel/spire#757