Fix testeability typo (#542)

Author: derekperkins

RULES_DESCRIPTIONS.md

@@ -136,7 +136,7 @@ _Configuration_: N/A
 ## cognitive-complexity
 
 _Description_: [Cognitive complexity](https://www.sonarsource.com/resources/white-papers/cognitive-complexity.html) is a measure of how hard code is to understand. 
-While cyclomatic complexity is good to measure "testeability" of the code, cognitive complexity aims to provide a more precise measure of the difficulty of understanding the code.
+While cyclomatic complexity is good to measure "testability" of the code, cognitive complexity aims to provide a more precise measure of the difficulty of understanding the code.
 Enforcing a maximum complexity per function helps to keep code readable and maintainable.
 
 _Configuration_: (int) the maximum function complexity