Add reference link for monotonic stack implementation

Added a comment with a reference link for monotonic stack.

Author: NayanSaraff

src/test/java/com/thealgorithms/stacks/MonotonicIncreasingStack.java

@@ -59,3 +59,5 @@ public static void main(String[] args) {
         System.out.println("Next Smaller Element: " + Arrays.toString(nextSmaller));
     }
 }
+
+/* https://www.geeksforgeeks.org/dsa/introduction-to-monotonic-stack-2/ */