fix small typo in guide-lifetimes

Author: emkay

src/doc/guide-lifetimes.md

@@ -77,7 +77,7 @@ value. We also call this _borrowing_ the local variable
 `on_the_stack`, because we have created an alias: that is, another
 name for the same data.
 
-Likewise, in the case of `owned_box`,
+Likewise, in the case of `on_the_heap`,
 the `&` operator is used in conjunction with the `*` operator
 to take a reference to the contents of the box.