I did not find a vendor having mistake-free documents. It is not a big issue, especially in background knowledge. It is a human work and exposed to such. I checked the capture here. The "255.255.255.128" is the wrong part. The next part is correct. You could send feedback directly to Fortinet.
Yep, typos are quite frequent in the documentation. You don't really fall for them with experience. The ones causing problem are not typos but bad explanation/wording leading to misunderstanding. The ones you have to read twice-trice to really understand what the author meant.