[benchmark] Fix lint in IsolateSpawn hello world.

Change-Id: I76495ccc6afecb141e7648b202ab46da0027b582
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/157260
Reviewed-by: Alexander Aprelev <aam@google.com>

benchmarks/IsolateSpawn/dart/helloworld.dart

@@ -1,3 +1,3 @@
-main() {
+void main() {
   print('Hello, world!');
 }

benchmarks/IsolateSpawn/dart2/helloworld.dart

@@ -1,3 +1,3 @@
-main() {
+void main() {
   print('Hello, world!');
 }