Update SUMMARY.md

SUMMARY.md

@@ -34,7 +34,8 @@
     * [vsyscall and vDSO](SysCall/syscall-3.md)
     * [How the Linux kernel runs a program](SysCall/syscall-4.md)
 * [Timers and time management](Timers/README.md)
-    * [Introduction](Timers/timers-1.md)	
+    * [Introduction](Timers/timers-1.md)
+    * [Clocksource framework](Timers/timers-2.md)
 * [Memory management](mm/README.md)
     * [Memblock](mm/linux-mm-1.md)
     * [Fixmaps and ioremap](mm/linux-mm-2.md)