Doesn't take the right argument to write the file in the dest folder

I used this configuration , and when I used the grunt task, it wrote a file named dest at the same path than my gruntfile

```
  pure_grids: {
            dest : 'src/css/vendors/purecss/grids/tva-grid.less',
            options: {
                units: 24, //24-column grid
                mediaQueries: {
                    sm : 'screen and (min-width: 35.5em)', // 568px
                    med: 'screen and (min-width: 48em)',   // 768px
                    lrg: 'screen and (min-width: 58em)',   // 928px
                    xl : 'screen and (min-width: 75em)'    // 1200px
                }
            }
        }
```