Fix Live Editing Zen Theme docs

[ch4og]

Since zen-browser/desktop#927 was merged starting from a.31 you won't be able to use Ctrl+Shift+Alt+I to open browser toolbox. First you will need to enable remote debugging (https://firefox-source-docs.mozilla.org/devtools-user/browser_toolbox/index.html)

[IAmJafeth]

Fixed: ca872e2