npm install the wrong grunt-docs plugin.

On www.npmjs.org the grunt-docs package is registered by shama's grunt-plugin https://github.com/shama/grunt-docs, but not the gruntjs team's grunt-plugin https://github.com/gruntjs/grunt-docs, so when I run `npm install` under the gruntjs.com project folder, npm install the wrong grunt-docs plugin, then when running the grunt task, grunt-docs goes wrong.  Correct this by specifying the grunt-docs tarball url !