x/website/tour: indicate if the displayed source code is modified

[hyangah]

It seems like go.dev/tour remembers modification in the playground from the past.
When visiting the pages again, it presents modified versions until the user clicks the "Reset" button.
This can be confusing. Consider to add a visual sign when presented code is a modified version.
(e.g. Reset button gets activated only when the code is dirty, etc)