Fix contributing install instruction

[sjvenditto]

Addresses issue #228

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.34%. Comparing base (a510ef3) to head (17f40ce).
Report is 207 commits behind head on development.

Additional details and impacted files

@@               Coverage Diff               @@
##           development     #309      +/-   ##
===============================================
+ Coverage        96.13%   97.34%   +1.20%     
===============================================
  Files               34       35       +1     
  Lines             2642     2862     +220     
===============================================
+ Hits              2540     2786     +246     
+ Misses             102       76      -26     

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[BalzaniEdoardo]

I love it, the number of reviewers is so random! anyone here wants to assign the review to Alex instead? 😂😂
...I know the policy originally required 2 approvals, but I'll relaxed it to one only.

First PR for @sjvenditto, more fun PRs to come! 🚀 🚀🚀

[BalzaniEdoardo]

I am a big fun of single quotations, but this time I will let this slide 😂