Update natural_language_understanding_v1.py

[manishth]

I was getting this error:

NameError: name 'features' is not defined

found out that line no. 15:
features=[features.Entities(), features.Keywords()])
should be:
features=[Features.Entities(), Features.Keywords()])

Notice the uppercase 'F'.

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All committers have signed the CLA.

[codecov-io]

Codecov Report

Merging #231 into master will decrease coverage by 2.82%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #231      +/-   ##
==========================================
- Coverage   84.31%   81.49%   -2.83%     
==========================================
  Files          24       24              
  Lines        1167     1167              
==========================================
- Hits          984      951      -33     
- Misses        183      216      +33

Impacted Files	Coverage Δ
watson_developer_cloud/language_translator_v2.py	44.82% <0%> (-31.04%)	⬇️
..._developer_cloud/watson_developer_cloud_service.py	63.54% <0%> (-9.38%)	⬇️
watson_developer_cloud/tone_analyzer_v3.py	92.3% <0%> (-7.7%)	⬇️
watson_developer_cloud/dialog_v1.py	38.88% <0%> (-5.56%)	⬇️
watson_developer_cloud/document_conversion_v1.py	93.75% <0%> (-3.13%)	⬇️

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