Update docs/book/src/reference/watching-resources.md

Co-authored-by: Erik Mogensen <erik@mogensoft.net>

docs/book/src/reference/watching-resources.md

@@ -100,7 +100,7 @@ and **Secondary Resources** remains the same:
 Therefore, regardless of whether the resource was defined by your project or by another project,
 your controller can watch, reconcile, and manage changes to these resources as needed.
 
-## Why watch the secondary resources matters?
+## Why does watching the secondary resources matter?
 
 When building a Kubernetes controller, it’s crucial to not only focus
 on **Primary Resources** but also to monitor **Secondary Resources**.