Clean up errors in Developer Guide

[lordidiot]

Closes #125

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e4687d8) 82.52% compared to head (f03d7d2) 82.52%.
Report is 4 commits behind head on master.

❗ Current head f03d7d2 differs from pull request most recent head b3b12a1. Consider uploading reports for the commit b3b12a1 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #126   +/-   ##
=========================================
  Coverage     82.52%   82.52%           
  Complexity      742      742           
=========================================
  Files            99       99           
  Lines          2300     2300           
  Branches        257      257           
=========================================
  Hits           1898     1898           
  Misses          337      337           
  Partials         65       65           

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[conradsoon]

LGTM, thanks for catching this mistake.