Fixed example in documentation

Added the cost of the edge 3 -> 4 on the example in the module documentation

Author: jdferreira

src/libcollections/binary_heap.rs

@@ -112,7 +112,7 @@
 //!     //                  7
 //!     //          +-----------------+
 //!     //          |                 |
-//!     //          v   1        2    |
+//!     //          v   1        2    |  2
 //!     //          0 -----> 1 -----> 3 ---> 4
 //!     //          |        ^        ^      ^
 //!     //          |        | 1      |      |