refactor(docs-infra): fix docs examples for tslint rule prefer-const (#38143)

This commit updates the docs examples to be compatible with the
`prefer-const` tslint rule.

This is in preparation of updating the docs examples `tslint.json` to
match the one generated for new Angular CLI apps in a future commit.

PR Close #38143

aio/content/examples/user-input/src/app/click-me2.component.ts

@@ -12,7 +12,7 @@ export class ClickMe2Component {
   clicks = 1;
 
   onClickMe2(event: any) {
-    let evtMsg = event ? ' Event target is ' + event.target.tagName  : '';
+    const evtMsg = event ? ' Event target is ' + event.target.tagName  : '';
     this.clickMessage = (`Click #${this.clicks++}. ${evtMsg}`);
   }
 }