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/forms-overview/src/app/shared/utils.ts

@@ -1,5 +1,5 @@
 export function createNewEvent(eventName: string, bubbles = false, cancelable = false) {
-  let evt = document.createEvent('CustomEvent');
+  const evt = document.createEvent('CustomEvent');
   evt.initCustomEvent(eventName, bubbles, cancelable, null);
   return evt;
 }