Update proxy docs to include fetchApi

Signed-off-by: Alex McKay <77302540+alex-mckay@users.noreply.github.com>

docs/tutorials/using-backstage-proxy-within-plugin.md

@@ -114,20 +114,22 @@ look something like this:
 
 /* ... */
 
-import { DiscoveryApi } from '@backstage/core-plugin-api';
+import { DiscoveryApi, FetchApi } from '@backstage/core-plugin-api';
 
 export class MyAwesomeApiClient implements MyAwesomeApi {
   discoveryApi: DiscoveryApi;
+  fetchApi: FetchApi;
 
-  constructor({discoveryApi}: {discoveryApi: DiscoveryApi}) {
+  constructor({discoveryApi, fetchApi}: {discoveryApi: DiscoveryApi, fetchApi: FetchApi}) {
     this.discoveryApi = discoveryApi;
+    this.fetchApi = FetchApi;
   }
 
   private async fetch<T = any>(input: string, init?: RequestInit): Promise<T> {
     // As configured previously for the backend proxy
     const proxyUri = `${await this.discoveryApi.getBaseUrl('proxy')}/<your-proxy-uri>`;
 
-    const resp = await fetch(`${proxyUri}${input}`, init);
+    const resp = await this.fetchApi.fetch(`${proxyUri}${input}`, init);
     if (!resp.ok) throw new Error(resp);
     return await resp.json();
   }