Fix a typo "learnd" -> "learned".

SysCall/syscall-4.md

@@ -394,7 +394,7 @@ That's all. From this point our programm will be executed.
 Conclusion
 --------------------------------------------------------------------------------
 
-This is the end of the fourth and last part of the about the system calls concept in the Linux kernel. We saw almost all related stuff to the `system call` concept in these four parts. We started from the understanding of the `system call` concept, we have learnd what is it and why do users applications need in this concept. Next we saw how does the Linux handle a system call from an user application. We met two similar concepts to the `system call` concept, they are `vsyscall` and `vDSO` and finally we saw how does Linux kernel run an user program.
+This is the end of the fourth and last part of the about the system calls concept in the Linux kernel. We saw almost all related stuff to the `system call` concept in these four parts. We started from the understanding of the `system call` concept, we have learned what is it and why do users applications need in this concept. Next we saw how does the Linux handle a system call from an user application. We met two similar concepts to the `system call` concept, they are `vsyscall` and `vDSO` and finally we saw how does Linux kernel run an user program.
 
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