fix typo

Author: KennethKinLum

1-js/11-async/08-async-await/article.md

@@ -298,7 +298,7 @@ The `async` keyword before a function has two effects:
 
 The `await` keyword before a promise makes JavaScript wait until that promise settles, and then:
 
-1. If it's an error, the exception is generated — same as if `throw error` were called at that very place.
+1. If it's an error, an exception is generated — same as if `throw error` were called at that very place.
 2. Otherwise, it returns the result.
 
 Together they provide a great framework to write asynchronous code that is easy to both read and write.