MAINT: fix signature of BaseWrapper.scorer

[adriangb]

Closes #168

[codecov-io]

Codecov Report

Merging #169 (fc72b40) into master (312b144) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #169   +/-   ##
=======================================
  Coverage   99.55%   99.55%           
=======================================
  Files           6        6           
  Lines         679      679           
=======================================
  Hits          676      676           
  Misses          3        3           

Impacted Files	Coverage Δ
scikeras/wrappers.py	99.44% <100.00%> (ø)

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