Fix usage description for "versions" subcommand

Author: rwstauner

exe/shopify-ruby

@@ -19,7 +19,7 @@ when "env"
   puts %{export RUBY_BUILD_DEFINITIONS="#{definitions}"}
 else
   $stderr.puts <<~USAGE
-    Usage:  shopify-ruby list
+    Usage:  shopify-ruby versions
             shopify-ruby resolve <version>
             shopify-ruby build [-kpv] <definition> <prefix>
             shopify-ruby env