Page Chapter 7. Working with Nodes, Page 86
I do not know enough to determine whether it is an error. But you tell us to refer to Chapter 2 for "more information on creating .install files" where on pg. 11 you tell us that custom modules reside in the "sites/all/modules" subdirectory, however on pg. 86 you say to put the joke module in the "sites/all/modules/custom" directory. I do not know if one is an error and the other is not or if there is some basis for putting joke inside "custom" and leaving "annotate" one level above. Is this something to do with implementing a node, is it an alternative?
Again, the author must use his expertise to decide: If it is an error I would imagine that you either use the custom directory both places or you don't use the custom directory both places, or the author can include a sentence or two explaining the decision to do it differently on the two pages.
It is an unexplained inconsistency in directions for creating a module that *may* be an error.