Context2D: don't use an arrow to illustrate scaling in the docs.

The example image scales an arrow, but uses an arrow to indicate that
the next image (an arrow) is a result of the first arrow being scaled.
Instead, use a circle.

Change-Id: I8de18d4c2f3cb309681f63c31b0c9f7255d1868c
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>