Update README.md

README.md

 	* `String concatenation(int, String)`
 	* `String getPrefix(String)`
 	* `String getSuffix(String)`
-	* `String compareTwoStrings(String)`
+	* `Boolean compareTwoStrings(String)`
 	* `Character getMiddleCharacter(String)`
 	* `String getFirstWord(String)`
 	* `String getSecondWord(String)`