Fix autoload path for RipperParser

[ksss]

Description

The autoload specification is incorrect and should be corrected. The class YARD::Parser::Ruby::Legacy::RipperParser does not exist and is assumed to be YARD::Parser::Ruby::RipperParser.

YARD::Parser::Ruby::Legacy.constants
#=> [:Statement, :RipperParser, :RubyParser, :RubyToken, :StatementList, :TokenList]
YARD::Parser::Ruby::Legacy::RipperParser
#=> uninitialized constant YARD::Parser::Ruby::Legacy::RipperParser (NameError)

Completed Tasks

• [x] I have read the Contributing Guide.
• [x] The pull request is complete (implemented / written).
• [x] Git commits have been cleaned up (squash WIP / revert commits).
• [x] I wrote tests and ran bundle exec rake locally (if code is attached to PR).