Fix show_banner config item (#820)

the config option to disable banner is show_banner, not show-banner
That does not leave a great impression when the first thing read in
documentation is incorrect...

Co-authored-by: Dominique Martinet <dominique.martinet@atmark-techno.com>

Author: martinetd

book/configuration.md

@@ -69,7 +69,7 @@ You can learn more about setting up colors and theming in the [associated chapte
 
 ## Remove Welcome Message
 
-To remove the welcome message, you need to edit your `config.nu` by typing `config nu` in your terminal, then you go to the global configuration `let-env config` and set `show-banner` option to false, like this:
+To remove the welcome message, you need to edit your `config.nu` by typing `config nu` in your terminal, then you go to the global configuration `let-env config` and set `show_banner` option to false, like this:
 
 @[code](@snippets/installation/remove_welcome_message.nu)
 

snippets/installation/remove_welcome_message.nu

@@ -1,5 +1,5 @@
 let-env config = {
   ...
-  show-banner: false,
+  show_banner: false,
   ...
 }