Fix indents for multi-line docstrings

[Dominic-Walther]

Docstrings spanning multiple lines now get properly indented.
I added a copy of api.yaml with multi-line descriptions to avoid making changes to 28 other tests that depend on api.yaml.

[codecov]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (6bf50ab) compared to base (e9b6edf).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##            master      #938     +/-   ##
===========================================
  Coverage   100.00%   100.00%             
===========================================
  Files           11        21     +10     
  Lines         1020      2603   +1583     
  Branches       201       614    +413     
===========================================
+ Hits          1020      2603   +1583     

Flag	Coverage Δ
unittests	99.92% <99.90%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
datamodel_code_generator/model/__init__.py	100.00% <ø> (ø)
datamodel_code_generator/__init__.py	100.00% <100.00%> (ø)
datamodel_code_generator/__main__.py	100.00% <100.00%> (ø)
datamodel_code_generator/format.py	100.00% <100.00%> (ø)
datamodel_code_generator/http.py	100.00% <100.00%> (ø)
datamodel_code_generator/imports.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/base.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/enum.py	100.00% <100.00%> (ø)
...atamodel_code_generator/model/pydantic/__init__.py	100.00% <100.00%> (ø)
...amodel_code_generator/model/pydantic/base_model.py	100.00% <100.00%> (ø)
... and 18 more

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[koxudaxi]

I'm sorry for my late reply.
Thank you for creating the PR.
Looks good :)