docs: fix init-hoot script for hostname matching

The init-hook provided for creating a distrobox with the
the same hostname as its host has two typos:

- Command substitution wrapping `uname -n` suppressing stdout redirect
- trailing "`"

Using this init-hook as suggested causes build errors.
This fix corrects the suggested script by echoing the command
substitution and removing the trailing "`".

See issue 89luca89#1164 for details.

docs/useful_tips.md

@@ -222,7 +222,7 @@ creating it with the following init-hook:
 
 ```sh
 distrobox create --name test --image your-chosen-image:tag \
-                  --init-hooks '"$(uname -n)" > /etc/hostname'`
+                  --init-hooks 'echo "$(uname -n)" > /etc/hostname'
 ```
 
 This will ensure SSH X-Forwarding will work when SSH-ing inside the distrobox: