chore: update godoc address (gin-gonic#2357)

README.md

@@ -5,7 +5,7 @@
 [![Build Status](https://travis-ci.org/gin-gonic/gin.svg)](https://travis-ci.org/gin-gonic/gin)
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 [![Go Report Card](https://goreportcard.com/badge/github.com/gin-gonic/gin)](https://goreportcard.com/report/github.com/gin-gonic/gin)
-[![GoDoc](https://godoc.org/github.com/gin-gonic/gin?status.svg)](https://godoc.org/github.com/gin-gonic/gin)
+[![GoDoc](https://godoc.org/github.com/gin-gonic/gin?status.svg)](https://pkg.go.dev/github.com/gin-gonic/gin?tab=doc)
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