.gitignore paths are matching too many things. Afix paths to root of agov profile

When making a new sub-theme, the vendor folder is properly copied over, but you can't add it to Git because the .gitignore is preventing _any_ folder named "vendor" to be added. In fact, we only intended /vendor to be excluded from Git.

So we need to prefix our paths with `/` to make them specific to the root of the aGov profile directory.