docs(adding-an-rss-feed): Fix incorrect filename (gatsbyjs#16290)

docs/docs/adding-an-rss-feed.md

@@ -63,7 +63,7 @@ The good news is you can accommodate these scenarios and more in `gatsby-config.
 
 To customize the default feed schema (a.k.a. structure) output by the plugin to work with your website's content, you can start with the following code:
 
-```js:title=gatsby.config.js
+```js:title=gatsby-config.js
 module.exports = {
   plugins: [
     {
@@ -140,7 +140,7 @@ To see your feed in action, run `gatsby build && gatsby serve` and you can then
 
 If creating a RSS feed for a podcast you probably will want to include iTunes RSS blocks. They take the format of `itunes:author` which GraphQL does not read. Here's an example of how to implement iTunes RSS blocks using this plugin:
 
-```js:title=gatsby.config.js
+```js:title=gatsby-config.js
 module.exports = {
   plugins: [
     {