Align arrows in diagrams (#39)

book/src/03_ticket_v1/09_heap.md

@@ -82,7 +82,7 @@ s.push_str("Hey");
       +---------+--------+----------+
 Stack | pointer | length | capacity |
       |  |      |   3    |    5     |
-      +----|----+--------+----------+
+      +--|  ----+--------+----------+
          |
          |
          v

book/src/03_ticket_v1/10_references_in_memory.md

@@ -28,7 +28,7 @@ you'll get something like this in memory:
       +----v----+--------+----------+      +----|----+
 Stack | pointer | length | capacity |      | pointer |
       |  |      |   3    |    5     |      |         |
-      +----|----+--------+----------+      +---------+
+      +--|  ----+--------+----------+      +---------+
          |          s                           r
          |
          v