Fix mainClassName to point to correct class

Author: ampan98

build.gradle

@@ -48,7 +48,7 @@ application {
 }
 
 shadowJar {
-    mainClassName = "duke"
+    mainClassName = "duke.Duke"
     archiveBaseName = "duke"
     archiveClassifier = null
 }
@@ -59,4 +59,4 @@ checkstyle {
 
 run{
     standardInput = System.in
-}
+}