Fix grammatical error

The sentence should say "Why we need tests" with no question mark, or "Why do we need tests?" I think keeping the question mark makes it more personal.

Author: tamiolaf

1-js/03-code-quality/05-testing-mocha/article.md

@@ -2,7 +2,7 @@
 
 Automated testing will be used in further tasks, and it's also widely used in real projects.
 
-## Why we need tests?
+## Why do we need tests?
 
 When we write a function, we can usually imagine what it should do: which parameters give which results.