Update documentation link

[Aaron Boranian]

Current documentation reference in Euclid menu window is for "Legacy OpenStudio User Guide". This is a link to an HTML file that is part of the Euclid install, but references Legacy OpenStudio.

This should be updated to remove references to Legacy OpenStudio.

[Peter Ellis]

For the time being, Euclid is still sufficiently similar to the Legacy OpenStudio extension that the Legacy User Guide is still the best reference documentation. And we don't have anything else yet. When the functionality in Euclid changes significantly, it will become necessary to rework the User Guide. Until then, we'll put this issue "on hold".