Switch to the recommended URL for the old Google Chart API

Note that this code uses the old API which is officially deprecated,
however converting this to use the new Chart API will take some time.

Obtained from:	https://developers.google.com/chart/image/docs/making_charts
Missed in:	r45633


git-svn-id: svn+ssh://svn.freebsd.org/doc/head@45681 c2e8774f-c49f-e111-b436-862b2bbc8956