Clarify different versions of `asdf install`

help.txt

@@ -15,9 +15,9 @@ asdf plugin update --all                Update all plugins
 MANAGE PACKAGES
 asdf install                            Install all the package versions listed
                                         in the .tool-versions file 
-asdf install [<name>]                   Install one tool at the version
+asdf install <name>                     Install one tool at the version
                                         specified in the .tool-versions file
-asdf install [<name> <version>]         Install a specific version of a package
+asdf install <name> <version>           Install a specific version of a package
 asdf install <name> latest[:<version>]  Install the latest stable version of a
                                         package, or with optional version,
                                         install the latest stable version that