Update README.md

Author: Mariana Azevedo

README.md

@@ -1,16 +1,16 @@
-![Hackerrank logo](src/main/resources/logo.png)
+<p align="center"><img src="src/main/resources/logo.png" alt="Hackerrank logo" width="450" align="center"/></p><br/>
 
-# hackerrank-challenges
+## hackerrank-challenges
 
 A project of solved exercises from Hackerrank website in Java and SQL.
 
 ## Summary
 
-### Java Challenges
+#### Java Challenges
 
 - [Interview Preparation Kit](https://github.com/mariazevedo88/hackerrank-challenges/blob/master/src/main/java/io/github/mariazevedo88/hc/prepkit/INDEX.md)
 - [Problem Solving](https://github.com/mariazevedo88/hackerrank-challenges/blob/master/src/main/java/io/github/mariazevedo88/hc/problems/INDEX.md)
 
-### SQL Challenges
+#### SQL Challenges
 
 - [Practice SQL](https://github.com/mariazevedo88/hackerrank-challenges/blob/master/sql/SQL.md)