Path to existing docs.css
Path to existing docs.css

Without this change, `grunt` fails with the following:

    >> Attempting to parse...OK
    >> FileError: 'vendor/bootstrap/docs/assets/css/docs.css' wasn't found in docs/assets/css/assemble.css on line 6, column 1:
    >> 5 @import (less) "src/less/bootstrap/print.less";
    >> 6 @import (less) "vendor/bootstrap/docs/assets/css/docs.css";//
    >> 7 // Scaffolding
    Warning: Error compiling LESS. Use --force to continue.

doowb/assemble-docs,holidayextras/assemble.io