Update blog cover image path

I have updated the blog cover image path for the blog at 'difference-between-stringbuffer-and-stringbuilder'

Author: gauravkukade

_posts/java-string/2019-12-24-difference-between-stringbuffer-and-stringbuilder.md

@@ -2,7 +2,7 @@
 layout: post
 title: "Difference Between `StringBuffer` and `StringBuilder` class"
 author: gaurav
-image: assets/images/2019-12-24/cover.jpg
+image: assets/images/2019-12-24-difference-between-stringbuffer-and-stringbuilder/blog-cover.jpg
 categories: [ Java, Core Java, String]
 description: "In this article, we are going to discuss the differences between the `StringBuffer` and `StringBuilder` class. And we will see why it `StringBuilder` is preferable to `StringBuffer` class."