dispatch: use the right context manager to deactivate demandimport

In e86d12404d69 I very embarrassingly wrote a patch with the
completely wrong function name. This should fix it.

mercurial/dispatch.py

@@ -181,7 +181,7 b' def _runcatch(req):'
                     debugtrace[debugger] == debugtrace['pdb']):
                     ui.warn(_("%s debugger specified "
                               "but its module was not found\n") % debugger)
-                with demandimport.disabled():
+                with demandimport.deactivated():
                     debugtrace[debugger]()
             try:
                 return _dispatch(req)