knife exec docs need an update

[coderanger]

https://docs.chef.io/knife_exec.html has #{n.memory.total} in an example, that needs to be #{n["memory"]["total"]} or similar.

[callen-IO]

This still needs to be updated since this will break a knife exec command if using a v13 or newer Chef client.

After downgrading to an older ChefDK version and running the same command, I was able to see this when using #{n.memory.total}:

WARN: method access to node attributes (node.foo.bar) is deprecated and will be removed in Chef 13, please use bracket syntax (node["foo"]["bar"]) (CHEF-4) at -E Argument:in `block in run'.
Please see https://docs.chef.io/deprecations_attributes.html for further details and information on how to correct this problem.

From the deprecation page that is referenced:

Setting and accessing node attributes has been standardized on “bracket” syntax. The older “method” syntax is deprecated and will be removed in Chef 13.