Use currentStep() instead of _currentStep in docs

Author: vkrol

docs/docs/intro/api.md

@@ -372,7 +372,7 @@ If you want to prevent displaying the next / previous step, return `false` in `p
 
 ```javascript
 introJs().onbeforechange(function() {
-  alert("before new step. Step: " + this._currentStep);
+  alert("before new step. Step: " + this.currentStep());
 
   // return false to stop displaying the next step
   // return false;

example/callbacks/onbeforechange.html

@@ -72,7 +72,7 @@ <h4>Section Six</h4>
       function startIntro(){
         var intro = introJs();
         intro.onbeforechange(function () {
-          if (this._currentStep === 2) {
+          if (this.currentStep() === 2) {
             alert('You cannot continue! :P')
             return false;
           }