Update api-changes.md

Fix a typo in the docs

docs/book/src/component-config-tutorial/api-changes.md

@@ -14,7 +14,7 @@ steps](/quick-start.md#installation) before continuing.
 kubebuilder init --domain tutorial.kubebuilder.io --component-config
 ```
 
-## Setting up an exising project
+## Setting up an existing project
 
 If you've previously generated a project we can add support for parsing the
 config file by making the following changes to `main.go`.