DOC: update docs to reflect default hash_method value

[ariekahn]

Looks like hash_method has defaulted to timestamp for a while:
https://github.com/nipy/nipype/blob/master/nipype/utils/config.py

Updating the config documentation.

[codecov-io]

Codecov Report

Merging #1792 into master will increase coverage by -0.03%.

@@            Coverage Diff             @@
##           master    #1792      +/-   ##
==========================================
- Coverage   69.83%   69.81%   -0.03%     
==========================================
  Files        1047     1047              
  Lines       52050    52050              
  Branches     7661     7651      -10     
==========================================
- Hits        36349    36338      -11     
- Misses      14123    14134      +11     
  Partials     1578     1578

Flag	Coverage Δ
#unittests	69.81% <ø> (-0.03%)	❌

Impacted Files	Coverage Δ
nipype/pipeline/plugins/pbs.py	19.11% <ø> (-1.48%)	❌
nipype/pipeline/plugins/condor.py	14.7% <ø> (-1.48%)	❌
nipype/pipeline/plugins/lsf.py	14.86% <ø> (-1.36%)	❌
nipype/pipeline/plugins/slurm.py	14.66% <ø> (-1.34%)	❌
nipype/pipeline/plugins/oar.py	29.06% <ø> (-1.17%)	❌
nipype/interfaces/mrtrix/convert.py	21.73% <ø> (-0.55%)	❌
nipype/pipeline/plugins/sge.py	17.74% <ø> (-0.44%)	❌
nipype/interfaces/base.py	79.98% <ø> (-0.37%)	❌

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