This is not true.
Once the battery is fully charged, you'll notice the icon in the upper-right changes from the lightning bolt to a plug. That means the phone is running on power straight from the outlet. You can't "overcharge" the battery.
For a more in-depth explanation of all things iPhone battery, see this thread: https://forums.macrumors.com/threads/525741/
What I'd it shows the plugin sign and my percentage is at 98%? It has went down from 100% to 98% and I've left it plugged in.