Update double-free.md

A slight typo here probably. Printing i1 would always result in the same address regardless of the double free.

binary-exploitation/libc-heap/double-free.md

@@ -89,7 +89,7 @@ int main() {
     printf("g1: %p\n", (void *)g1);
     printf("h1: %p\n", (void *)h1);
     printf("i1: %p\n", (void *)i1);
-    printf("i2: %p\n", (void *)i1);
+    printf("i2: %p\n", (void *)i2);
 
     return 0;
 }