docs:fix label name for kube_pod_nodeselectors

[yosshi825]

What this PR does / why we need it:
The docs says nodeselectors_NODE_SELECTOR as follows:

nodeselectors_NODE_SELECTOR=<NODE_SELECTOR>

ref : https://github.com/kubernetes/kube-state-metrics/blob/master/docs/pod-metrics.md

Check the metric in kms v 2.5.0 and the label name is nodeselector_ NODE _ SELECTOR

# HELP kube_pod_nodeselectors Describes the Pod nodeSelectors.
# TYPE kube_pod_nodeselectors gauge
kube_pod_nodeselectors{namespace="kube-system",pod="node-local-dns-mpm2x",uid="0f954293-170b-4f0c-91f3-bf4b4e334ce2",nodeselector_role="worker"} 1

 # actual  metrics
- nodeselector_NODE_SELECTOR
 # docs description
+ nodeselectors_NODE_SELECTOR

How does this change affect the cardinality of KSM: (increases, decreases or does not change cardinality)

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #1751

[mrueg]

/lgtm

[k8s-ci-robot]

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This pull-request has been approved by: mrueg, yosshi825

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