Fix small typo on first_steps.md (denoland#732)

getting_started/first_steps.md

@@ -148,7 +148,7 @@ import { serve } from "https://deno.land/std@$STD_VERSION/http/server.ts";
 
 const handler = async (_request: Request): Promise<Response> => {
   const resp = await fetch("https://api.github.com/users/denoland", {
-    // The init object here has an headers object containing a
+    // The init object here has a headers object containing a
     // header that indicates what type of response we accept.
     // We're not specifying the method field since by default
     // fetch makes a GET request.