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

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.