Update rustup_toolchain_cmd_help_flag_stdout.toml

Author: kyle-leonhard

tests/suite/cli-ui/rustup/rustup_toolchain_cmd_help_flag_stdout.toml

@@ -28,7 +28,9 @@ Discussion:
 
         <channel>[-<date>][-<host>]
 
-        <channel>       = stable|beta|nightly|<major.minor>|<major.minor.patch>
+        <channel>       = stable|beta|nightly|<versioned>[-<beta tag>]
+        <versioned>     = <major.minor>|<major.minor.patch>
+        <beta tag>      = beta[.<number>]
         <date>          = YYYY-MM-DD
         <host>          = <target-triple>