Use ret to return from main instead of interrupting to exit program

We are linking against a C library anyways.

README.md

@@ -64,9 +64,7 @@ main:
         call    exit
         add     esp,4
 
-        mov     eax,1
-        mov     ebx,0
-        int     0x80
+        ret
 
 SECTION .data
 msg     db      `Hello World! ebx = ?\n`,0