docs: Fix accepted Provider type in doc snippet

out of the box my IDE tells me there an error on

    const testProviders: Provider[] = [provideHttpClient(), provideHttpClientTesting()];

because `provideHttpClient()` returns an `EnvironmentProviders` so
I can't put it in a variable of type `Provider[]`

adev/src/content/guide/testing/overview.md

@@ -53,11 +53,10 @@ The `setupFiles` and `providersFile` options are particularly useful for managin
 For example, you could create a `src/test-providers.ts` file to provide `provideHttpClientTesting` to all your tests:
 
 ```typescript {header: "src/test-providers.ts"}
-import {Provider} from '@angular/core';
-import {provideHttpClient} from '@angular/common/http';
+import {EnvironmentProviders, Provider} from '@angular/core';
 import {provideHttpClientTesting} from '@angular/common/http/testing';
 
-const testProviders: Provider[] = [provideHttpClient(), provideHttpClientTesting()];
+const testProviders: (Provider | EnvironmentProviders)[] = [provideHttpClientTesting()];
 
 export default testProviders;
 ```