Docs: Fix the script in the example to actually clone the template contents, not the template element itself.

Review URL: https://codereview.chromium.org/680673002

I had to also register for the text/plain mime-type
which turned out to be harder than expected.
Mostly due to my confusion with mojo_shell
using last-argument-wins argument parsing.

R=ianh@google.com

Review URL: https://codereview.chromium.org/672363002

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/677513002