Fix example of URLMap in MarkdownOpts struct

[budougumi0617]

I changed comment URLMap in MarkdownOpts.
Usually, the package management tools(dep, glide etc) creates subdirectory named hostname under the vendor directory.

$ tree vendor -L  1
vendor
├── github.com
└── golang.org

But, the example of URLMap does not include hostname in PATH, I think correct mapping is github.com/my/package/vendor/github.com/go-chi/chi/.

I would appreciate it if you could check this PR.