FIX: Change plt.hist() argument from deprecated 'normed' to 'density'

[sjshim]

Summary

Fixes #3451 .

[codecov]

Codecov Report

Merging #3455 (a6f929f) into master (3dc858e) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master    #3455   +/-   ##
=======================================
  Coverage   65.24%   65.24%           
=======================================
  Files         309      309           
  Lines       40833    40833           
  Branches     5376     5376           
=======================================
  Hits        26641    26641           
  Misses      13118    13118           
  Partials     1074     1074           

Flag	Coverage Δ
unittests	65.02% <0.00%> (ø)

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Impacted Files	Coverage Δ
nipype/algorithms/metrics.py	56.28% <0.00%> (ø)

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[effigies]

Thanks!