Docs for arc: origin -> center

Issue description

The description has the following sentence:

"The origin of the arc's ellipse may be changed with the ellipseMode() function. "

This should be:

"The center of the arc's ellipse may be changed with the ellipseMode() function. "

The word "center" is used for ellipses. The word "origin" is used for coordinate systems.
Reference: https://en.wikipedia.org/wiki/Ellipse

URL(s) of affected page(s)

https://processing.org/reference/arc_.html

Proposed fix

Replace "origin" with "center".

打开 https://processing.org/reference/arc_.html 中受影响的 arc 参考页面，并找到描述椭圆原点的句子。按提议将“origin”替换为“center”，然后查看渲染后的页面，以确认措辞正确。