docs(rxfire): fix the table layout on auth.md (#2396)

The pipe character (`|`) in the return value of `idToken()` was breaking the table. The pipe is now escaped properly for Markdown.

Author: CreativeBuilds

packages/rxfire/docs/auth.md

@@ -62,7 +62,7 @@ The `idToken()` function creates an observable that emits the `idToken` refreshe
 | **function**    | `idToken()`                                 |
 | **params**      | `auth.Auth`                              |
 | **import path** | `rxfire/auth`                            |
-| **return**      | `Observable<string | null>`              |
+| **return**      | `Observable<string\|null>`              |
 
 #### TypeScript Example
 ```ts