fix(docs): show where getNodesByType comes from (#25344)

* show where getNodesByType comes from

* indicate the code is an example per PR feedback

Co-authored-by: Marcy Sutton <marcy@gatsbyjs.com>

Co-authored-by: Marcy Sutton <marcy@gatsbyjs.com>

docs/docs/creating-a-source-plugin.md

@@ -429,10 +429,10 @@ One challenge when developing locally is that a developer might make modificatio
 
 If possible, the proactive listener approach creates the best experience if existing APIs in the data source can support it (or you have access to build support into the data source).
 
-The code to support this behavior looks like this:
+Here's some pseudo code that shows this behavior:
 
 ```javascript:title=source-plugin/gatsby-node.js
-exports.sourceNodes = async ({ actions }, pluginOptions) => {
+exports.sourceNodes = async ({ actions, getNodesByType }, pluginOptions) => {
   const { createNode, touchNode, deleteNode } = actions
 
   // highlight-start