tuned message, example is in NEWS;

--- a/src/Pure/Tools/dotnet_setup.scala	Sat Nov 05 22:59:38 2022 +0100
+++ b/src/Pure/Tools/dotnet_setup.scala	Sat Nov 05 23:08:35 2022 +0100
@@ -166,10 +166,6 @@
   See also:
     https://fsharp.org
     https://learn.microsoft.com/en-us/dotnet/core/tools/dotnet-install-script
-
-  Example:
-    isabelle dotnet_setup
-    isabelle dotnet fsi
 """,
           "D:" -> (arg => target_dir = Path.explode(arg)),
           "I:" -> (arg => install_url = arg),