docs(alert-controller): use const instead of let (#12964)

src/components/alert/alert-controller.ts

@@ -51,12 +51,10 @@ import { Config } from '../../config/config';
  * ```ts
  * import { AlertController } from 'ionic-angular';
  *
- * constructor(private alertCtrl: AlertController) {
- *
- * }
+ * constructor(public alertCtrl: AlertController) { }
  *
  * presentAlert() {
- *   let alert = this.alertCtrl.create({
+ *   const alert = this.alertCtrl.create({
  *     title: 'Low battery',
  *     subTitle: '10% of battery remaining',
  *     buttons: ['Dismiss']
@@ -65,7 +63,7 @@ import { Config } from '../../config/config';
  * }
  *
  * presentConfirm() {
- *   let alert = this.alertCtrl.create({
+ *   const alert = this.alertCtrl.create({
  *     title: 'Confirm purchase',
  *     message: 'Do you want to buy this book?',
  *     buttons: [
@@ -88,7 +86,7 @@ import { Config } from '../../config/config';
  * }
  *
  * presentPrompt() {
- *   let alert = this.alertCtrl.create({
+ *   const alert = this.alertCtrl.create({
  *     title: 'Login',
  *     inputs: [
  *       {
@@ -181,14 +179,14 @@ import { Config } from '../../config/config';
  * out first, *then* start the next transition.
  *
  * ```ts
- * let alert = this.alertCtrl.create({
+ * const alert = this.alertCtrl.create({
  *   title: 'Hello',
  *   buttons: [{
  *     text: 'Ok',
  *     handler: () => {
  *       // user has clicked the alert button
  *       // begin the alert's dismiss transition
- *       let navTransition = alert.dismiss();
+ *       const navTransition = alert.dismiss();
  *
  *       // start some async method
  *       someAsyncOperation().then(() => {