readme: fix manual install instructions
readme: fix manual install instructions

Before:

```
> go get -u github.com/scaleway/c14-cli/pkg/cmd
package github.com/scaleway/c14-cli/pkg/cmd: cannot find package "github.com/scaleway/c14-cli/pkg/cmd" in any of:
```

After:

```
> go get -u github.com/scaleway/c14-cli/cmd/c14
> cd $GOPATH/src/github.com/scaleway/c14-cli
> ls
assets	cmd  Dockerfile  glide.lock  glide.yaml  LICENSE.md  Makefile  pkg  README.md  vendor
```

Signed-off-by: Stanislas Lange <f35d6940585b099132cd6f6603ab778759397762@pm.me>

online-net/c14-cli