[Therion] survey-level: Minor correction /clarification of entry in Therion Book

[Martin Budaj]

> _ survey-level <N/all> . how many surveys to display with stations.
>
> Does it control the display of the nested level of survey?  Eg 126 versus
> 126 at f.Greenlink.RiwakaSystem

Exactly. N should be numeric value, default value is 0.

Martin