fix punctuation in list (again)

Author: rutsky

src/doc/trpl/mutability.md

@@ -85,8 +85,8 @@ philosophy, memory safety, and the mechanism by which Rust guarantees it, the
 > You may have one or the other of these two kinds of borrows, but not both at
 > the same time:
 > 
-> * one or more references (`&T`) to a resource.
-> * exactly one mutable reference (`&mut T`)
+> * one or more references (`&T`) to a resource,
+> * exactly one mutable reference (`&mut T`).
 
 [ownership]: ownership.html
 [borrowing]: references-and-borrowing.html#borrowing