Update docs/cluster_and_device_type_dev/how_to_add_new_dts_and_clusters.md

Author: rbultman

docs/cluster_and_device_type_dev/how_to_add_new_dts_and_clusters.md

@@ -186,7 +186,7 @@ related SDK tests.
     1. [cluster_test_plan_template.adoc](https://github.com/CHIP-Specifications/chip-test-plans/blob/master/src/template/cluster_test_plan_template.adoc)
     2. [test_plan_template.adoc](https://github.com/CHIP-Specifications/chip-test-plans/blob/master/src/template/test_plan_template.adoc)
 
-    Also, ask to join the private `csg-tt-test-plans` Slack channel.
+    Also, ask to be added to the private `csg-tt-test-plans` Slack channel.
 
 5. Note: the CHIP-Tool reference client is generated from XML
 6. If applicable, add tests: