helm: fix typo in prometheus README.md
helm: fix typo in prometheus README.md

Changes `merics` to `metrics`

Signed-off-by: Bhavin Gandhi <3d8021411d571aa893a0c72c9f69289a4b6d8690@gmail.com>

jescarri/prometheus-operator,coreos/prometheus-operator,jescarri/prometheus-operator,coreos/prometheus-operator