Fix grafana parse errors

[jbfavre]

2 small typos cause Grafana parse error on panels using theses expressions

[jbfavre]

Also update README.md
after running jb install github.com/metalmatze/kube-thanos/jsonnet/kube-thanos@master, ./build example.jsonnet gave me this error:

+ rm -rf manifests
+ mkdir manifests
+ jsonnet -J vendor -m manifests example.jsonnet
+ xargs '-I{}' sh -c 'cat {} | gojsontoyaml > {}.yaml; rm -f {}' -- '{}'
RUNTIME ERROR: field does not exist: variables
	vendor/kube-thanos/kube-thanos-querier.libsonnet:26:68-86	thunk <image>
	vendor/ksonnet/ksonnet.beta.4/k8s.libsonnet:22402:51-56	thunk <image>
	vendor/ksonnet/ksonnet.beta.4/k8s.libsonnet:22422:46-51	object <anonymous>
	vendor/kube-thanos/kube-thanos-store.libsonnet:(85:17)-(91:20)	thunk <array_element>
	vendor/kube-thanos/kube-thanos-store.libsonnet:(84:27)-(92:16)	object <anonymous>
	vendor/kube-thanos/kube-thanos-store.libsonnet:(82:22)-(94:12)	object <anonymous>
	vendor/kube-thanos/kube-thanos-store.libsonnet:(81:16)-(95:10)	object <anonymous>
	vendor/kube-thanos/kube-thanos-store.libsonnet:(80:20)-(96:8)	object <anonymous>
	example.jsonnet:30:1-104	object <anonymous>
	During manifestation

Using jb install github.com/thanos-io/kube-thanos/jsonnet/kube-thanos@master fixed the issue

[kakkoyun]

@jbfavre Thank you very much for catching this. We're currently working on the migration of thanos-mixin to Thanos itself, please see related PRs #67 and thanos-io/thanos#1760.

I have added your changes with this commit thanos-io/thanos@4d95064.

Therefore, I'm closing this one. Please feel free to comment on related PRs if you see any other issues.