Update writing-tests-for-phpunit.rst

sebastianbergmann#225

Author: ricfio

src/writing-tests-for-phpunit.rst

@@ -485,14 +485,14 @@ See :numref:`writing-tests-for-phpunit.data-providers.examples.DependencyAndData
             return [['provider1'], ['provider2']];
         }
 
-        public function testProducerFirst(): void
+        public function testProducerFirst(): string
         {
             $this->assertTrue(true);
 
             return 'first';
         }
 
-        public function testProducerSecond(): void
+        public function testProducerSecond(): string
         {
             $this->assertTrue(true);