Update install for go get/go install in README

If you try to install the tool, you currently get:

```
$ go install github.com/fullstorydev/grpcurl/cmd/grpcurl
go: finding github.com/cncf/udpa/go latest
build github.com/fullstorydev/grpcurl/cmd/grpcurl: cannot load github.com/cncf/udpa/go/udpa/core/v1: module github.com/cncf/udpa/go@latest (v0.0.0-20201120205902-5459f2c99403) found, but does not contain package github.com/cncf/udpa/go/udpa/core/v1
```

I tried using the latest tagged version as well via `go get`, but the same issue exists there as well. This is a downstream mod issue with https://github.com/cncf/udpa not apparently having a version tag, but they have since updated that it appears.

```
$ go get -u -v github.com/fullstorydev/grpcurl/cmd/grpcurl@v1.7.0
go: finding github.com v1.7.0
go: finding github.com/fullstorydev/grpcurl/cmd/grpcurl v1.7.0
go: finding github.com/fullstorydev/grpcurl/cmd v1.7.0
go: finding github.com/fullstorydev v1.7.0
go: finding github.com/cncf/udpa/go latest
go: finding google.golang.org/genproto latest
go: finding golang.org/x/net latest
go: finding golang.org/x/sys latest
go: finding golang.org/x/oauth2 latest
build github.com/fullstorydev/grpcurl/cmd/grpcurl: cannot load github.com/cncf/udpa/go/udpa/core/v1: module github.com/cncf/udpa/go@latest (v0.0.0-20201120205902-5459f2c99403) found, but does not contain package github.com/cncf/udpa/go/udpa/core/v1
```

Review the installation instructions in README and compare the documented go get/go install commands with the failure shown in this issue. Confirm the updated instructions avoid the reported github.com/cncf/udpa/go package error and that the documented command installs grpcurl successfully.

Written by the indexing model from the issue text.