pdb: use plural form of resource name in allowlist key
pdb: use plural form of resource name in allowlist key

All other allowlist selectors are using plural form of resource name
except PDB. Actually it is a typo introduced by PR[1].

[1] https://github.com/kubernetes/kube-state-metrics/pull/1623

Signed-off-by: Arunprasad Rajkumar <e2e07c768d0243d7afb2847cadecb44d327acf1b@redhat.com>

kubernetes/kube-state-metrics,kubernetes/kube-state-metrics