SUMMARY: fix typo in the timers chapter

SUMMARY.md

@@ -40,7 +40,7 @@
     * [Introduction to timers](Timers/timers-4.md)
     * [Clockevents framework](Timers/timers-5.md)
     * [x86 related clock sources](Timers/timers-6.md)
-    * [Time realted syetem calls](Timers/timers-7.md)
+    * [Time related system calls](Timers/timers-7.md)
 * [Memory management](mm/README.md)
     * [Memblock](mm/linux-mm-1.md)
     * [Fixmaps and ioremap](mm/linux-mm-2.md)