Fix example call to Octokit.label

[garethrees]

Before:

Octokit.labels("octokit/octokit.rb", "V3 Addition")
Traceback (most recent call last):
        7: from /usr/local/bin/irb:11:in `<main>'
        6: from (irb):6
        5: from /usr/local/lib/ruby/gems/2.5.0/gems/octokit-4.8.0/lib/octokit.rb:46:in `method_missing'
        4: from /usr/local/lib/ruby/gems/2.5.0/gems/octokit-4.8.0/lib/octokit/client/labels.rb:19:in `labels'
        3: from /usr/local/lib/ruby/gems/2.5.0/gems/octokit-4.8.0/lib/octokit/connection.rb:79:in `paginate'
        2: from /usr/local/lib/ruby/gems/2.5.0/gems/octokit-4.8.0/lib/octokit/connection.rb:185:in `parse_query_and_convenience_headers'
        1: from /usr/local/lib/ruby/gems/2.5.0/gems/octokit-4.8.0/lib/octokit/connection.rb:185:in `delete'
TypeError (no implicit conversion of Symbol into String)

After:

Octokit.label("octokit/octokit.rb", "V3 Addition")
=> {:id=>111852, :url=>"https://api.github.com/repos/octokit/octokit.rb/labels/v3%20addition", :name=>"v3 addition", :color=>"ededed", :default=>false}

[tarebyte]

Thanks ✨

[tarebyte]

This has been released as part of https://github.com/octokit/octokit.rb/releases/tag/v4.9.0