docs: Fixed wrong command for robots_txt CLI task

This is according to the error message displayed when trying to run the
command in the current version of the docs

docs/docs/administration/CLI_tasks/robots_txt.md

@@ -11,11 +11,11 @@ If you want to generate a restrictive `robots.txt`, you can run the following mi
 === "OTP"
 
     ```sh
-    ./bin/pleroma_ctl robotstxt disallow_all
+    ./bin/pleroma_ctl robots_txt disallow_all
     ```
 
 === "From Source"
 
     ```sh
-    mix pleroma.robotstxt disallow_all
+    mix pleroma.robots_txt disallow_all
     ```