Optimized the code a bit.

src/helloworld.asm

@@ -1,6 +1,8 @@
 .section .data
 hello:
-        .ascii "Hello, World.\n"    # the string
+    .ascii "Hello, World!\n"    # the string
+hello_end:
+.equ hello_len, hello_end - hello
 
 .section .text
     .global _start
@@ -9,9 +11,9 @@ _start:
     mov $1, %rax                    # Define write syscall
     mov $1, %rdi                    # Define stdout
     mov $hello, %rsi                # Give string
-    mov $14, %rdx                   # Give size of the string
+    mov $hello_len, %rdx                   # Give size of the string
     syscall                         # Call the kernel to do work for us
 
     mov $60, %rax                   # Define exit syscall
-    xor %rdi, %rdi                  # Give return value
+    mov $0, %rdi                    # Give return value
     syscall                         # Call the kernel to do work for us