rpk: adds avaiable language for app generate (#1426)

Author: paulohtb6

modules/reference/pages/rpk/rpk-generate/rpk-generate-app.adoc

@@ -16,7 +16,7 @@ By default, this runs interactively, prompting you to select a language and
 a user with which to create your application. To use this without interactivity,
 specify how you want your application to be created using flags.
 
-The `--language` flag lets you specify the language. There is no default.
+The `--language` flag lets you specify the language. There is no default. Available language: `go`.
 
 The `--new-sasl--user` flag lets you generate a new SASL user
 with admin ACLs. If you don't want to use your current profile user or don't want to create a
@@ -73,7 +73,7 @@ rpk generate app [flags]
 |-h, --help |- |Help for app.
 
 |-l, --language |string |The language you want the code sample to be
-generated with.
+generated with. Available language: `go`.
 
 |--new-sasl-credentials |string |If provided, `rpk` will generate and use
 these credentials (<user>:<password>).