Commit cee110b2 (IPython Notebook: Fixing coding crimes for section
names) changed IPython notebook section name to use '-' as word
delimiter but forgot to update users, and this way wendelin build
started to fail:

    INFO While:
    INFO   Installing.
    INFO   Getting section ipython_notebook.
    INFO Error: The referenced section, 'ipython_notebook', was not defined.

Fix it. (And I've made sure with whole-tree git grep that there is no
more ipython notebook users except wendelin in whole slapos.git so far)

/reported-by @Tyagov
/cc @tiwariayush
/reviewed-by TrustMe