fix: fix prism highlight signature

The new typings define higligth as follow:

```typescript
/*
 * @param text A string with the code to be highlighted.
 * @param grammar An object containing the tokens to use.
 *
 * Usually a language definition like `Prism.languages.markup`.
 * @param language The name of the language definition passed to `grammar`.
 * @returns The highlighted HTML.
 *
 * @example
 * Prism.highlight('var foo = true;', Prism.languages.js, 'js');
 */
export function highlight(
	text: string,
	grammar: Grammar,
	language: string
): string;
```

projects/demo/src/app/prism.service.ts

@@ -8,6 +8,6 @@ import 'prismjs/components/prism-typescript';
 export class PrismService {
 
   highlight(code: string, lang: string) {
-    return prism.highlight(code.trim(), prism.languages[lang]);
+    return prism.highlight(code.trim(), prism.languages[lang], lang);
   }
 }