UI: Set replay buffer check w/ --startreplaybuffer

When starting the program with the --startreplaybuffer command line
parameter, it would not correctly highlight the button.

UI/window-basic-main.cpp

@@ -5365,6 +5365,7 @@ void OBSBasic::ReplayBufferStart()
 		return;
 
 	replayBufferButton->setText(QTStr("Basic.Main.StopReplayBuffer"));
+	replayBufferButton->setChecked(true);
 
 	if (sysTrayReplayBuffer)
 		sysTrayReplayBuffer->setText(replayBufferButton->text());