Fix negation in Algebraic decoder

[LukaJCB]

The tests didn't find this issue due to typelevel/spire#757 :(

[codecov-io]

Codecov Report

Merging #6 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master       #6   +/-   ##
=======================================
  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% <100%> (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 9b3b186...689aac7. Read the comment docs.