Fix typo (#25)

Author: HairyFotr

sections/4.2-ownership.tex

@@ -186,7 +186,7 @@ \subsection{Promise 2: No dangling pointers}
 println!("New value is {}", s);
 \end{minted}
 
-While borrow a mutable reference to a value, that refrence is the only way to
+While borrow a mutable reference to a value, that reference is the only way to
 access that value at all.
 \end{frame}