Fix Texts fr Sequence Diagram

Author: Tusamma Sal Sabil

object-oriented-design-case-studies/design-stack-overflow.md

@@ -100,6 +100,8 @@ Here are the main classes of Stack Overflow System:
 
 ### Sequence Diagram
 
+Following is the sequence diagram for creating a new question:
+
 <p align="center">
  <img src="/media-files/stack-overflow-sequence-diagram.svg" alt="Stack Overflow Sequence Diagram">
  <br />