Typo in wtf.html: a adress book -> an adress book

docs/intro/wtf.rst

@@ -20,7 +20,7 @@ up in to features of the system.
 After enumerating features, you need to create scenarios which will
 describe those feature. Thus, scenarios are components of a feature.
 
-Let's learn by example: suppose we want to create a system to manage a
+Let's learn by example: suppose we want to create a system to manage an
 address book.
 
 OK, one of the very basic features of a address book is adding contacts, which will