Docs for triangle

### Issue description

The docs for `triangle` says:

"A triangle is a plane created by connecting three points. "

This is confusing since a plane is infinite.

How about changing "plane" to "shape" (to match the explanation of a rectangle
in the documentation for `rect`)?


### URL(s) of affected page(s)

https://processing.org/reference/triangle_.html

### Proposed fix

