Fix a malformed CHECKPOINT_M

It uses %s, but there is no variable passed to it, so it literally prints "Driver %s successfully made", this fixes that.

src/RageFileManager.cpp

@@ -725,7 +725,7 @@ bool RageFileManager::Mount( const RString &sType, const RString &sRoot_, const
 		return false;
 	}
 
-	CHECKPOINT_M("Driver %s successfully made.");
+	CHECKPOINT_M(ssprintf("Driver %s successfully made.", sType.c_str()));
 
 	LoadedDriver *pLoadedDriver = new LoadedDriver;
 	pLoadedDriver->m_pDriver = pDriver;