Data for test coverage report is too old

[htynkn]

https://github.com/apache/incubator-dubbo-docs/blob/cf0f72963389c370a34cba4864db93938996003b/dubbo-user-book/coveragence.md

it says image was generated at 2012-02-03

[ralf0131]

Yes, maybe we should add a link to the latest coverage report?

[htynkn]

@ralf0131 yeah link is better. i also think once the work to improve unit test coverage is done, we can capture new screenshot for coverage