all: add or change GoDoc badge to pkg.go.dev with update-readmes.go

update-readmes.go is updated to add a pkg.go.dev badge instead of a
godoc.org badge.

The GoDoc badge is added or changed in all READMEs by running
update-readmes.go

For golang/go#42251

Change-Id: I0c49df26a6c695f938c98c9a7b26325d991b3cfb
Reviewed-on: https://go-review.googlesource.com/c/build/+/276032
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/cmd/gitmirror/README.md b/cmd/gitmirror/README.md
index e6584e2..7fd0728 100644
--- a/cmd/gitmirror/README.md
+++ b/cmd/gitmirror/README.md
@@ -1,7 +1,7 @@
 <!-- Auto-generated by x/build/update-readmes.go -->
 
-[![GoDoc](https://godoc.org/golang.org/x/build/cmd/gitmirror?status.svg)](https://godoc.org/golang.org/x/build/cmd/gitmirror)
+[![Go Reference](https://pkg.go.dev/badge/golang.org/x/build/cmd/gitmirror.svg)](https://pkg.go.dev/golang.org/x/build/cmd/gitmirror)
 
 # golang.org/x/build/cmd/gitmirror
 
-The gitmirror binary watches the specified Gerrit repositories for new commits and reports them to the build dashboard.
+The gitmirror binary watches the specified Gerrit repositories for new commits and syncs them to GitHub.