fixed sample bootloader code

linux-bootstrap-1.md

@@ -109,9 +109,9 @@ boot:
     mov ah, 0x0e
     mov bh, 0x00
     mov bl, 0x07
-    mov al, !
+    mov al, '!'
 
-	int 0x10
+    int 0x10
     jmp $
 
 times 510-($-$$) db 0