doc: Fix import url in readme

[Bzil]

doc: Fix import url in readme
error :

❯ go install
main.go:10:8: no required module provides package github.com/mirakl/lib-vault/vaultv2; to add it:
        go get github.com/mirakl/lib-vault/vaultv2
❯ go get github.com/mirakl/lib-vault/vaultv2

go: module github.com/mirakl/lib-vault@upgrade found (v1.1.1), but does not contain package github.com/mirakl/lib-vault/vaultv2

fix:

 go get github.com/mirakl/lib-vault/v2
go: downloading github.com/mirakl/lib-vault/v2 v2.4.1
go: downloading golang.org/x/time v0.5.0
go: downloading github.com/go-test/deep v1.1.0
go: downloading github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc
go: downloading github.com/hashicorp/hcl v1.0.1-vault-5
go: downloading github.com/hashicorp/go-secure-stdlib/parseutil v0.1.8
go: downloading github.com/pmezard/go-difflib v1.0.1-0.20181226105442-5d4384ee4fb2
go: downloading github.com/hashicorp/go-sockaddr v1.0.6
go: upgraded github.com/hashicorp/go-secure-stdlib/parseutil v0.1.6 => v0.1.8
go: upgraded github.com/hashicorp/go-sockaddr v1.0.2 => v1.0.6
go: upgraded github.com/hashicorp/hcl v1.0.0 => v1.0.1-vault-5
go: added github.com/mirakl/lib-vault/v2 v2.4.1
go: upgraded golang.org/x/time v0.0.0-20200416051211-89c76fbcd5d1 => v0.5.0