fix(docs): change plnkr form to open in same window

[jeffbcross]

Form previously posted to target="_blank", but pop-up blockers were causing this to not work.
If a user chose to bypass pop-up blocker one time and click the link, they would arrive at
a new default plnkr, not a plnkr with the desired template.

This fix removes the _blank target, causing the plnkr to open in the current window/tab.

[mary-poppins]

Thanks for the PR! Please check the items below to help us merge this faster. See the contributing docs for more information.

• Uses the issue template (#8327)

If you need to make changes to your pull request, you can update the commit with git commit --amend.
Then, update the pull request with git push -f.

Thanks again for your help!

[IgorMinar]

not ideal. but let's do it.

[jeffbcross]

I think it only gets blocked because the submission does not happen in context of a user event (the click). It happens after an xhr call to load the data to submit. The ideal user experience would be to have the forms pre-populated and ready to submit on click. I'll add an issue to the backlog.