Rework the second chapter

.gitignore

@@ -53,3 +53,4 @@ dkms.conf
 
 # Our binaries
 hello/hello
+sum/sum

README.md

@@ -22,7 +22,7 @@ Have fun!
 Here are links to each post:
 
   * [Part 1. Introduction](https://github.com/0xAX/asm/blob/master/content/asm_1.md)
-  * [Say hello to x86_64 Assembly part 2](https://github.com/0xAX/asm/blob/master/content/asm_2.md)
+  * [Part 2. The `x86_64` concepts](https://github.com/0xAX/asm/blob/master/content/asm_2.md)
   * [Say hello to x86_64 Assembly part 3](https://github.com/0xAX/asm/blob/master/content/asm_3.md)
   * [Say hello to x86_64 Assembly part 4](https://github.com/0xAX/asm/blob/master/content/asm_4.md)
   * [Say hello to x86_64 Assembly part 5](https://github.com/0xAX/asm/blob/master/content/asm_5.md)

content/asm_1.md

@@ -165,7 +165,7 @@ _start:
     syscall
     ;; Specify the number of the system call (60 is `sys_exit`).
     mov    rax, 60
-    ;; Set the first argument of `sys_exit` to `0`. The `0` status code is success.
+    ;; Set the first argument of `sys_exit` to 0. The 0 status code is success.
     mov    rdi, 0
     ;; Call the `sys_exit` system call.
     syscall