Fix a Note name in CmmNode

("Continuation BlockIds" is referenced in CmmProcPoint)

[skip ci]

compiler/cmm/CmmNode.hs

@@ -114,7 +114,7 @@ data CmmNode e x where
       cml_cont :: Maybe Label,
           -- Label of continuation (Nothing for return or tail call)
           --
-          -- Note [Continuation BlockId]: these BlockIds are called
+          -- Note [Continuation BlockIds]: these BlockIds are called
           -- Continuation BlockIds, and are the only BlockIds that can
           -- occur in CmmExprs, namely as (CmmLit (CmmBlock b)) or
           -- (CmmStackSlot (Young b) _).