"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' });
}
}
```

開啟 docs/functions/functions.md 並檢查 AngularFireFunctions 範例。對照回報的 TypeScript 錯誤檢查範本插值和 AppComponent 宣告，然後驗證修正後的範例可以編譯，並符合文件中所述的可呼叫函式用法。

由索引模型根據 Issue 內容生成。