Bug 1836150 - Adjust treeherder doc to use sheriffs channel instead of treeherder channel (#7708)

* Fixed pre-commit black bug

* Bug 1836150 - Adjust treeherder doc to use sheriffs channel instead of treeherder channel

* Fixed black to python 3.9

* Removed code from another bug

docs/testcases.md

@@ -41,6 +41,6 @@ Test the following:
 
 ## Ask Sheriffs
 
-1. Ping with the word `sheriff` in the `#treeherder` channel and request them
+1. Ping with the word `sheriff` in the `#sheriffs` channel and request them
    to test their workflows.
 2. Test around any high-risk commits since last deploy.