Fixes incorrect grammar in react/prefer-stateless-function docs

Author: corydolphin

docs/rules/prefer-stateless-function.md

@@ -1,6 +1,6 @@
 # Enforce stateless React Components to be written as a pure function (prefer-stateless-function)
 
-Stateless functional components are more simple than class based components and will benefit from future React performance optimizations specific to these components.
+Stateless functional components are simpler than class based components and will benefit from future React performance optimizations specific to these components.
 
 ## Rule Details