Fix incorrect comment in machine-scheduler

[javed.absar]

Summary

The example code incorrectly invokes ScheduleDAGMI wherein from context it is clear it intends to invoke ScheduleDAGMILive actually.

[atrick]

LGTM

include/llvm/CodeGen/MachineScheduler.h

 // scheduling while updating the instruction stream, register pressure, and live
 // intervals. Most targets don't need to override the DAG builder and list
 // schedulier, but subtargets that require custom scheduling heuristics may
 // plugin an alternate MachineSchedStrategy. The strategy is responsible for
 // selecting the highest priority node from the list:
 //
 // ScheduleDAGInstrs *<Target>PassConfig::
 // createMachineScheduler(MachineSchedContext *C) {
-//   return new ScheduleDAGMI(C, CustomStrategy(C));
+//   return new ScheduleDAGMILive(C, CustomStrategy(C));
 // }
 //
 // The DAG builder can also be customized in a sense by adding DAG mutations
 // that will run after DAG building and before list scheduling. DAG mutations
 // can adjust dependencies based on target-specific knowledge or add weak edges
 // to aid heuristics:
 //
 // ScheduleDAGInstrs *<Target>PassConfig::