Fix/docstring warnings 2

[Moritz-Alexander-Kern]

This PR fixes mostly indentation errors in several doc-strings. The content of the documentation is not affected.

Before:
when building the documentation with:

cd doc && make html

miscellaneous warnings appeared, most of them indentation errors.

With this PR:

• fixed indentations

Only 4 Warnings remain:

elephant/doc/reference/spade.rst:13: WARNING: duplicate label "Stella, 2019" for keys "citations-Stella2019_104022" and "spade-Stella2019_104022"
elephant/doc/reference/spike_train_generation.rst:43: WARNING: duplicate label "Staude, 2010" for keys "cubic-Staude2010_327" and "generation-Staude2010_327"
elephant/doc/reference/spike_train_surrogates.rst:12: WARNING: duplicate label "Louis, 2010" for keys "surrogates-Louis2010_127" and "surrogates-Louis2010_359"
elephant/doc/reference/unitary_event_analysis.rst:20: WARNING: duplicate label "Gr{\"u}n, 2002" for keys "unitary_event_analysis-Gruen02_43" and "unitary_event_analysis-Gruen02_81"

See log: Warnings_log_8_make html_after_fix.txt

[coveralls]

Coverage: 87.996%. Remained the same when pulling d6971c5 on INM-6:fix/docstring_warnings into a8aab43 on NeuralEnsemble:master.

[Moritz-Alexander-Kern]

build wheels