Fix documentation (noted in issue #116)

Author: facontidavide

docs/tutorial_04_sequence_star.md

@@ -156,9 +156,11 @@ would be _interrupted_ (_halted_, to be specific).
      <BehaviorTree>
         <ReactiveSequence>
             <BatteryOK/>
-            <SaySomething   message="mission started..." />
-            <MoveBase       goal="1;2;3"/>
-            <SaySomething   message="mission completed!" />
+            <Sequence>
+                <SaySomething   message="mission started..." />
+                <MoveBase       goal="1;2;3"/>
+                <SaySomething   message="mission completed!" />
+            </Sequence>
         </ReactiveSequence>
      </BehaviorTree>
  </root>

include/behaviortree_cpp_v3/bt_factory.h

@@ -156,8 +156,8 @@ struct Tree
 
     void haltTree()
     {
-        // the halt should propagate to all the node if all the nodes
-        // are implemented correctly
+        // the halt should propagate to all the node if the nodes
+        // have been implemented correctly
         root_node->halt();
         root_node->setStatus(NodeStatus::IDLE);