Update tag-pruning.md
Update tag-pruning.md

Added link to Go's time parser function
docker/docker.github.io,docker/docker.github.io,londoncalling/docker.github.io,thaJeztah/docker.github.io,thaJeztah/docker.github.io,thaJeztah/docker.github.io,londoncalling/docker.github.io,thaJeztah/docker.github.io,docker/docker.github.io,docker/docker.github.io,londoncalling/docker.github.io,docker/docker.github.io,londoncalling/docker.github.io,londoncalling/docker.github.io,thaJeztah/docker.github.io