fix binding for "bar" key in step editing bindings

It is not possible to generate "bar" without the shift modifier on most (all?)
keyboards. The same condition exists for the _ key in the main bindings

gtk2_ardour/step_editing.bindings

@@ -55,6 +55,6 @@
      <Binding key="apostrophe" action="StepEditing/toggle-triplet"/>
      <Binding key="period" action="StepEditing/toggle-dotted"/>
      <Binding key="Primary-period" action="StepEditing/no-dotted"/>
-     <Binding key="bar" action="StepEditing/toggle-chord"/>
+     <Binding key="Tertiary-bar" action="StepEditing/toggle-chord"/>
   </Press>
 </Bindings>