The Hyperspec is a copyrighted document owned by Lispworks. They can change it, and perhaps others could create a revised version under the generous terms of use.

But the Hyperspec is mostly a version of the official ANSI specification for CL, and revising that is something much more difficult. (I won't here go into the story how X3J13 manipulated the final draft of the ANS into a freely-available document prior to ANSI slapping their restrictive copyright on it, but that's what the Hyperspec and other similar freely-available versions of the CL specification are derived from.)

The erroneous example you found is real and it would certainly be nice to fix it, but note 1.4.3 Sections Not Formally Part Of This Standard. Sections labeled Examples or Notes are not normative -- i.e., they are illustrative only and not a binding part of the language definition.