Fix nested array parsing

[dduugg]

Description

Prior to this change, nested arrays would be missing closing brackets when translating back to source code, e.g.:

[1] pry(main)> YARD::Parser::Ruby::RubyParser.new('a[b[c]]', nil).parse.root.first.source
=> "a[b[c]"

This in turn would print warnings in YARD output, and result in documentation errors. Example warnings:

[warn]: Syntax error in `(stdin)`:(1,96): syntax error, unexpected end-of-input, expecting ']'
[warn]: ParserSyntaxError: syntax error in `(stdin)`:(1,96): syntax error, unexpected end-of-input, expecting ']'

This change corrects this specific problem. All tests pass, but this is a naïve fix, and I'm not familiar enough with this code to contemplate other potential side effects.

Completed Tasks

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

[lsegal]

Sorry it took so long to get to this. Thanks for the fix!