minor book fixes (#392)

Author: molpopgen

book/src/prelude.md

@@ -2,6 +2,6 @@
 
 The contents of the crate prelude are:
 
-```rust, noplaygound, ignore
+```rust, noplayground, ignore
 {{#include ../../src/prelude.rs}}
 ```

book/src/working_with_tree_sequences.md

@@ -4,18 +4,22 @@ The next several sections provide examples based on a tree sequence with two tre
 
 The first tree is:
 
+```md
      0
     +++
     | |  1
     | | +++
     2 3 4 5
+```
 
 The second tree is:
 
+```md
          0
        +-+-+
        1   |
      +-+-+ |
      2 4 5 3
+```