MINOR: update comments in config/producer.properties

Author: Manikumar Reddy <manikumar.reddy@gmail.com>

Reviewers: Ismael Juma <ismael@juma.me.uk>

Closes #4070 from omkreddy/prodcuer.config

(cherry picked from commit f5341c9f35)
Signed-off-by: Ismael Juma <ismael@juma.me.uk>

config/producer.properties

@@ -12,7 +12,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-# see org.apache.kafka.clients.producer for more details
+# see org.apache.kafka.clients.producer.ProducerConfig for more details
 
 ############################# Producer Basics #############################