Unreplace of assign_map should be to g:vimrplugin_assign_map

Behavior was not correct in the case that vimrplugin_assign_map is set
to something other than "(underscore)".  "Un"replacing vimrplugin_assign_map was
using "(underscore)" instead of the vimrplugin_assign_map string.

(underscore is written out because of markdown)