_content/blog: fix unavailable link

Change-Id: I8ef3c7d60f72c20c21e80609738772f3805a4c18
GitHub-Last-Rev: ea811c700a
GitHub-Pull-Request: golang/website#132
Reviewed-on: https://go-review.googlesource.com/c/website/+/387074
Reviewed-by: DO NOT USE <iant@google.com>
Trust: Dmitri Shuralyov <dmitshur@google.com>

_content/blog/go12.md

@@ -39,7 +39,7 @@ package.
 A major new feature of the tool chain is the facility to compute and display
 [test coverage results](/doc/go1.2#cover).
 See the [`go test`](/cmd/go/#hdr-Description_of_testing_flags)
-and [cover tool](https://godoc.org/code.google.com/p/go.tools/cmd/cover/)
+and [cover tool](https://golang.org/x/tools/cmd/cover)
 documentation for details. Later this week we will publish an article that
 discusses this new feature in detail.