Missing reference on page 207

**Describe the erratum**
Reference 18 regarding the Nyström approximation appears to be missing. The references in the right-hand margin jump straight from 17 to 19.

**Version and Location**
Page 207. It is missing both in the printed book and in the version I downloaded off the web page today. 

![Image](https://github.com/user-attachments/assets/26183604-18a0-4f2c-b7c0-f78125d33a11)