fix doc for unsafe_default_initialized

Summary: Fix a typo in the spelling of the name.

Reviewed By: dmm-fb

Differential Revision: D47303200

fbshipit-source-id: eff5706bfa0d0453b56fb1ab84a071eacc62bfe4

folly/Utility.h

@@ -439,8 +439,8 @@ using EnableCopyMove = std::conditional_t<
 using moveonly_::MoveOnly;
 using moveonly_::NonCopyableNonMovable;
 
-//  unsafe_default_uninitialized
-//  unsafe_default_uninitialized_cv
+//  unsafe_default_initialized
+//  unsafe_default_initialized_cv
 //
 //  An object which is explicitly convertible to any default-constructible type
 //  and which, upon conversion, yields a default-initialized value of that type.