[#230] Fix typo in Developer Guide

[zhenglong1603]

Fixes #230

Developer Guide: add missing "are"

The DeveloperGuide.md file is missing the word "are" in a sentence, which makes the sentence grammatically incorrect.

Let's add the missing word to improve documentation accuracy and readability.

[canihasreview]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.26%. Comparing base (9b27751) to head (9e42a8d).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #235   +/-   ##
=========================================
  Coverage     75.26%   75.26%           
  Complexity      419      419           
=========================================
  Files            71       71           
  Lines          1338     1338           
  Branches        126      126           
=========================================
  Hits           1007     1007           
  Misses          301      301           
  Partials         30       30           

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[damithc]

Thanks for the fix, @zhenglong1603