grunt.util deprecated functions should be indicated in the tutorial

Hi, I was looking at grunt.util.spawn today and I did find the way to use it here on the official website:
https://gruntjs.com/api/grunt.util#grunt.util.spawn
However, I got weird errors getting spawn to work (I was trying to execute some cmd).
After reading a bunch related topics on stackoverflow I finally ran into: https://github.com/gruntjs/grunt-legacy-util which says clearly in the doc that grunt.util.spawn is deprecated (along with some other grunt.util functions) and we should use node child_process instead.
Just think this should be mentioned in the official tutorial page otherwise it's misleading for users.

Thanks!

Start with the official tutorial and the grunt.util API page linked in the issue, then compare their guidance with the grunt-legacy-util documentation. Update the tutorial so the deprecated grunt.util functions, including spawn, are identified and users are directed toward Node's child_process; done when the warning is clear and the documentation no longer misleading.

Written by the indexing model from the issue text.