tutorial: pointer -> box

doc/tutorial.md

@@ -1233,8 +1233,8 @@ avoiding expensive copies.
 
 Rust has three "realms" in which objects can be allocated: the stack,
 the local heap, and the exchange heap. These realms have corresponding
-pointer types: the borrowed pointer (`&T`), the shared pointer (`@T`),
-and the unique pointer (`~T`). These three sigils will appear
+pointer types: the borrowed pointer (`&T`), the shared box (`@T`),
+and the unique box (`~T`). These three sigils will appear
 repeatedly as we explore the language. Learning the appropriate role
 of each is key to using Rust effectively.