add missing comma

Author: vladak

proc.tex

@@ -124,7 +124,7 @@
 
 \begin{itemize}
 \item A process will enter a kernel mode either by a
-\emph{trap induced by the CPU} (page fault, unknown instruction, etc.)
+\emph{trap induced by the CPU} (page fault, unknown instruction, etc.),
 \emph{timer} (to invoke scheduler), \emph{interrupt} from a peripheral device,
 or synchronous trap (a standard library uses it to hand over the control to
 the kernel to service a \emph{system call}).