examples.md: Actually use `ctx` variable in Go example
examples.md: Actually use `ctx` variable in Go example

"List and manage containers" Go example:
`ctx := context.Background()` was defined but never used.
Follow the structure of the other examples instead.
docker/docker.github.io,docker/docker.github.io,docker/docker.github.io,thaJeztah/docker.github.io,thaJeztah/docker.github.io,thaJeztah/docker.github.io,thaJeztah/docker.github.io,docker/docker.github.io,thaJeztah/docker.github.io,docker/docker.github.io