Incorrect find shelf command instruction

[kmnyn]

Error description: actual find shelf command is different from the user guide.
Steps to reproduce:

1. Add a book called using add book command:
   add Dune / Frank Herbert / scifi
2. Enter find command:
   find shelf SCIF-0-0
3. Expected output:
   Found 1 book(s) with the shelf ID 'SCIF-0-0':
4. Dune by Frank Herbert (Genre: scifi, Shelf: SCIF-0-0)
5. Actual output:
   [ERROR] Invalid search criteria 'shelf'. Use 'title', 'author', 'genre', or 'id'.
   5: Correct output is given when key in:
   find id SCIF-0-0