README: Fix URLs of the official Go org pages

[sryoya]

Sorry for the PR without an issue 👋

README uses the old URLs of the Go Playground and Go Doc (They are still valid but redirected to the new ones now.).

godoc.org -> pkg.go.dev play.golang.org -> go.dev/play

[mwoss]

Hi! :D Sorry for the late reply 🙏 I would love to introduce those doc updates before the 1.4.0 release, could you rebase so I could merge it? :D