make code match comment

sysfile.c

@@ -12,7 +12,7 @@
 // Fetch the nth word-sized system call argument as a file descriptor
 // and return both the descriptor and the corresponding struct file.
 static int
-argfd(int argno, int *pfd, struct file **pf)
+argfd(int n, int *pfd, struct file **pf)
 {
   int fd;
   struct file *f;