[docs] Upgrade: wrong path for patternlab-foot

[fuhlig]

Number 3 in the Upgrade Guide

Copy ./source/_patterns/00-atoms/00-meta/_01-foot.mustache to ./source/_meta/_00-foot.mustache

It should still be _01-foot.mustache

// Template.php
$patternFootPath         = $metaDir.DIRECTORY_SEPARATOR."_01-foot.".$patternExtension;

Not sure if anyone stumpled upon this but caused quite some confusion for me as the PL-navigation etc. was not working.
Would have opened a pull-request but could not find the correct repository - not even sure the documentation is on github.

Thanks in advance.