Update Stack.md

Author: ankurjuneja

DataStructures/Stack.md

@@ -1,6 +1,6 @@
 **Evaluate Post fix expression**
 ```
-public int evalPostFixExp(String expr) {
+    public int evalPostFixExp(String expr) {
         if(expr.equals(""))
             return 0;
         Stack<Integer> stack = new Stack<>();