doc: change documentation badge to go.dev

README.md

@@ -3,7 +3,7 @@
 Go [geohash](https://en.wikipedia.org/wiki/Geohash) library offering encoding
 and decoding for string and integer geohashes.
 
-[![GoDoc Reference](http://img.shields.io/badge/godoc-reference-5272B4.svg?style=flat-square)](http://godoc.org/github.com/mmcloughlin/geohash)
+[![go.dev Reference](https://img.shields.io/badge/doc-reference-007d9b?logo=go&style=flat-square)](https://pkg.go.dev/github.com/mmcloughlin/geohash)
 [![Build status](https://img.shields.io/travis/mmcloughlin/geohash.svg?style=flat-square)](https://travis-ci.org/mmcloughlin/geohash)
 [![Coverage](https://img.shields.io/coveralls/mmcloughlin/geohash.svg?style=flat-square)](https://coveralls.io/r/mmcloughlin/geohash)
 [![Go Report Card](https://goreportcard.com/badge/github.com/mmcloughlin/geohash?style=flat-square)](https://goreportcard.com/report/github.com/mmcloughlin/geohash)