Adds missing "class" keyword in starter files.

- Especially in the earlier exercises, let's make it one less
  error that folks need to track down.

(fixes #54)

etl/src/main/java/Etl.java

-public Etl {
+public class Etl {
 }

nucleotide-count/src/main/java/DNA.java

-public DNA {
+public class DNA {
 }

word-count/src/main/java/WordCount.java

-public WordCount {
+public class WordCount {
 }