minor fixes: documentation

[giovana-morais]

i was reading the documentation to create a loader and i found some typos, so i just ran a spell-check in the docs folder and fixed them.

Changelog

• spell-check documentation files and fix typos
• contributing.rst:
  • fix pip install syntax to install optional dependencies (Test dependencies installation instructions #564)
  • remove --black flag from pytest command because it was not working

[codecov]

Codecov Report

Merging #600 (8fecc6b) into master (496eb4a) will increase coverage by 0.04%.
Report is 5 commits behind head on master.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #600      +/-   ##
==========================================
+ Coverage   96.96%   97.00%   +0.04%     
==========================================
  Files          58       58              
  Lines        6990     6987       -3     
==========================================
  Hits         6778     6778              
+ Misses        212      209       -3     

[guillemcortes]

Thanks for your contribution @giovana-morais! We will take a look at this soon.

[guillemcortes]

Thanks again @giovana-morais ! Approving and merging the PR.