Update hooks.md

Fixing example by adding `[` and `]` to provide list to hook

```
Error: nu::shell::invalid_config

  × Encountered 1 error(s) when updating config

Error: nu::shell::type_mismatch

  × Type mismatch at $env.config.hooks.env_change.PWD
   ╭─[]
 4 │         env_change: {
 5 │ ╭─▶         PWD: {
 6 │ │               code: 'print $"changing directory from ($before) to ($after)"'
 7 │ ├─▶         }
   · ╰──── expected list<any>, but got record<code: string>
 8 │         }
   ╰────
```

Author: bf

book/hooks.md

@@ -188,9 +188,9 @@ When defining a hook as a string, the `$before` and `$after` variables are set t
 ```nu
 $env.config = ($env.config | upsert hooks {
     env_change: {
-        PWD: {
+        PWD: [{
             code: 'print $"changing directory from ($before) to ($after)"'
-        }
+        }]
     }
 })
 ```