Improve error message for invalid parser_engine value

lib/rubocop/ast/processed_source.rb

@@ -29,7 +29,7 @@ def self.from_file(path, ruby_version, parser_engine: :parser_whitequark)
       def initialize(source, ruby_version, path = nil, parser_engine: :parser_whitequark)
         unless PARSER_ENGINES.include?(parser_engine)
           raise ArgumentError, 'The keyword argument `parser_engine` accepts ' \
-                               "`parser` or `parser_prism`, but `#{parser_engine}` was passed."
+                               "`:parser_whitequark` or `:parser_prism`, but `#{parser_engine.inspect}` was passed."
         end
 
         # Defaults source encoding to UTF-8, regardless of the encoding it has