[trivial] Fix typo in ksonnet-gen/README
[trivial] Fix typo in ksonnet-gen/README

Signed-off-by: Jessica Yuen <2e0161c0db81eee1490ff8caa0e1fa30ba9bbf16@gmail.com>

heptio/kube.libsonnet,ksonnet/ksonnet-lib,ksonnet/ksonnet-lib