[ruby/prism] Fix back reference for ruby_parser on Ruby 2.7

Symbol#name is only a thing since Ruby 3.0

ruby/prism@2de82b15fc

Author: Earlopain

lib/prism/translation/ruby_parser.rb

@@ -152,7 +152,7 @@ def visit_assoc_splat_node(node)
         # ^^
         # ```
         def visit_back_reference_read_node(node)
-          s(node, :back_ref, node.name.name.delete_prefix("$").to_sym)
+          s(node, :back_ref, node.name.to_s.delete_prefix("$").to_sym)
         end
 
         # ```