_content/doc/telemetry: use a solid background for dataflow diagram

The transparent background for dataflow.png did not work well with a
dark theme. Use a solid white background.

For golang/go#63883

Change-Id: I9836522d3411125037dd70502c0dfe690a786157
Reviewed-on: https://go-review.googlesource.com/c/website/+/570601
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Hyang-Ah Hana Kim <[email protected]>

Author: findleyr