Update controller.md

Lowercase 's' to prevent misleading APIS as a whole word.

Author: whoif

content/en/docs/concepts/architecture/controller.md

@@ -102,7 +102,7 @@ Other control loops can observe that reported data and take their own actions.
 In the thermostat example, if the room is very cold then a different controller
 might also turn on a frost protection heater. With Kubernetes clusters, the control
 plane indirectly works with IP address management tools, storage services,
-cloud provider APIS, and other services by
+cloud provider APIs, and other services by
 [extending Kubernetes](/docs/concepts/extend-kubernetes/) to implement that.
 
 ## Desired versus current state {#desired-vs-current}