Update README.md

fixed a typo: $4%x -> %4$x

binary-exploitation/format-strings/README.md

@@ -87,7 +87,7 @@ and you would read from the first to the forth param.
 Or you could do:
 
 ```c
-printf("$4%x")
+printf("%4$x")
 ```
 
 and read directly the forth.