doc: Fix syntax in cluster example code

doc/api/cluster.markdown

@@ -470,7 +470,7 @@ on the specified worker.
 
     cluster.fork().on('online', function() {
       // Worker is online
-    };
+    });
 
 ### Event: 'listening'
 
@@ -481,7 +481,7 @@ on the specified worker.
 
     cluster.fork().on('listening', function(address) {
       // Worker is listening
-    };
+    });
 
 ### Event: 'disconnect'
 
@@ -490,7 +490,7 @@ on the specified worker.
 
     cluster.fork().on('disconnect', function() {
       // Worker has disconnected
-    };
+    });
 
 ### Event: 'exit'
 
@@ -510,4 +510,4 @@ is terminated.  See [child_process event: 'exit'](child_process.html#child_proce
       } else {
         console.log("worker success!");
       }
-    };
+    });