pangram: standardize starter implementation

exercises/pangram/src/main/java/PangramChecker.java

 public class PangramChecker {
 
     public boolean isPangram(String input) {
-        throw new UnsupportedOperationException("Method has not been implemented yet.");
+        throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
     }
+
 }