Fix example in UPDATING.md (#11518)

`my_plguin.MyCustomExecutor` -> `my_plugin.MyCustomExecutor` as we mention in the sentence above that "plugin was
called `my_plugin` "

UPDATING.md

@@ -1002,7 +1002,7 @@ called `my_plugin` then your configuration looks like this
 
 ```ini
 [core]
-executor = my_plguin.MyCustomExecutor
+executor = my_plugin.MyCustomExecutor
 ```
 And now it should look like this:
 ```ini