Fix syntax error of example

Fix syntax error of example.

before

```rb
      # @example
      #       @api.put_wall_post("Hello there!", {
      #         "name" => "Link name"
      #         "link" => "http://www.example.com/",
      #         "caption" => "{*actor*} posted a new review",
      #         "description" => "This is a longer description of the attachment",
      #         "picture" => "http://www.example.com/thumbnail.jpg"
      #       })
```

after

```rb
      # @example
      #       @api.put_wall_post("Hello there!", {
      #         "name" => "Link name",
      #         "link" => "http://www.example.com/",
      #         "caption" => "{*actor*} posted a new review",
      #         "description" => "This is a longer description of the attachment",
      #         "picture" => "http://www.example.com/thumbnail.jpg"
      #       })
```