getPageUrl renamed getPagedUrl

The README.md file initially calls one of the helper methods "getPagedUrl".  However it's actually called getPageUrl in the code itself and the code example within the readme file.  "getPagedUrl" would be a safer option for now, to avoid a conflict with "getPageUrl" from docpad-plugin-services.

__Note__: This would break code using this plugin, but they could easily rename the function or we could workaround it by providing an alias?