Change findById(…) assert message to reflect the actual operation.

Original pull request: #345.

Author: AbassAdeyemi

src/main/java/org/springframework/data/keyvalue/core/KeyValueTemplate.java

@@ -47,6 +47,7 @@
  * @author Thomas Darimont
  * @author Mark Paluch
  * @author Mert Zeybekler
+ * @author Adeyemi Abass
  */
 public class KeyValueTemplate implements KeyValueOperations, ApplicationEventPublisherAware {
 
@@ -256,7 +257,7 @@ public <T> Iterable<T> findAll(Class<T> type) {
 	@Override
 	public <T> Optional<T> findById(Object id, Class<T> type) {
 
-		Assert.notNull(id, "Id for object to be inserted must not be null!");
+		Assert.notNull(id, "Id for object to be found must not be null!");
 		Assert.notNull(type, "Type to fetch must not be null!");
 
 		String keyspace = resolveKeySpace(type);