Quick start - can only use path@version syntax with 'go get'

The issue surfaced when attempting to perform the steps that are outlined in: https://grpc.io/docs/languages/go/quickstart/#prerequisites

During the installation of prerequisites following the instructions in order to get gRPC working with Golang when using the following command:

```
go install google.golang.org/protobuf/cmd/protoc-gen-go@v1.26
```

I was met with an error as follows:
```
package google.golang.org/protobuf/cmd/protoc-gen-go@v1.26: can only use path@version syntax with 'go get'
```

After performing the ```go get google.golang.org/protobuf/cmd/protoc-gen-go@v1.26``` command as suggested by the language tooling the issue went away. The same issue exists for the second installation command in the instructions:

```
go install google.golang.org/grpc/cmd/protoc-gen-go-grpc@v1.1
```

I am not sure if this is language version specific or not. The language version I use is: ```go version go1.15.1 windows/amd64```
I will send a PR with the change applied.