rna-transcription: standardize starter implementation

exercises/rna-transcription/src/main/java/RnaTranscription.java

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