convert plurals to singular

[koxudaxi]

convert plurals name to singular name for items

Related issue

#18

[codecov-io]

Codecov Report

Merging #20 into master will not change coverage.
The diff coverage is n/a.

@@         Coverage Diff          @@
##           master   #20   +/-   ##
====================================
  Coverage       0%    0%           
====================================
  Files           7     6    -1     
  Lines         100    59   -41     
====================================
+ Misses        100    59   -41

Impacted Files	Coverage Δ
datamodel_code_generator/model/base.py

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