Fix import path and godoc link in README
Fix import path and godoc link in README

Go modules expects releases greater than or equal to v2.0.0 to be
imported with a `/v2` suffix.  Without the `/v2` suffix, `go mod` is
unable to detect the latest version of emoji and saves a "+incompatible"
version in the user's go.mod file.

godoc.org is deprecated and will begin redirecting to pkg.go.dev in
early 2021.  Update link and badge to point to pkg.go.dev.
See https://blog.golang.org/godoc.org-redirect for more information.

kyokomi/emoji