Fix Javadoc warning for org.web3j.protocol.core.Batcher (#1323)

> Task :core:javadoc
/Users/kkn/src/github.com/web3j/web3j/core/src/main/java/org/web3j/protocol/core/Batcher.java:18: warning: no @return
    BatchRequest newBatch();
                 ^
1 warning

BUILD SUCCESSFUL in 5s

core/src/main/java/org/web3j/protocol/core/Batcher.java

@@ -14,6 +14,10 @@
 
 public interface Batcher {
 
-    /** Create a new {@link BatchRequest}. */
+    /**
+     * Create a new {@link BatchRequest}.
+     *
+     * @return New BatchRequest.
+     */
     BatchRequest newBatch();
 }