"AngularFireFunctions" documentation has two errors

The example code in the [AngularFireFunctions](https://github.com/angular/angularfire/blob/master/docs/functions/functions.md) documentation doesn't compile. Here's the provided code, with comments on the two lines that don't compile:

```ts
import { Component } from '@angular/core';
import { AngularFireFunctions } from '@angular/fire/compat/functions';

@Component({
selector: 'app-root',
template: `{ data$ | async }` // doesn't compile
})
export class AppComponent {
constructor(private fns: AngularFireFunctions) {
const callable = fns.httpsCallable('my-fn-name');
this.data$ = callable({ name: 'some-data' }); // doesn't compile
}
}
```

The `template` line needs double curly brackets:

```ts
template: `{{ data$ | async }}`
```

The next error is:

```bash
Property 'data$' does not exist on type 'AppComponent'.

6 template: `{{ data$ | async }}`,
```

I corrected this by making a variable `data$: any`.

This code now compiles:

```ts
import { Component } from '@angular/core';
import { AngularFireFunctions } from '@angular/fire/compat/functions';

@Component({
selector: 'app-component',
template: `{{ data$ | async }}`,
})
export class AppComponent {
data$: any;

constructor(private fns: AngularFireFunctions) {
const callable = fns.httpsCallable('my-fn-name');
this.data$ = callable({ name: 'some-data' });
}
}
```

docs/functions/functions.md を開き、AngularFireFunctions の例を確認します。報告された TypeScript エラーと照らし合わせて、テンプレートの補間と AppComponent の宣言を確認し、その後、修正された例がコンパイルでき、ドキュメントに記載された callable function の使用方法と一致することを検証します。

索引モデルが issue の本文から書いたものです。