fix error rate chart pluralization

[graphaelli]

Kibana version: 7.9.0 bc @df6530c4531c6b1fccfaef1e07bf18d38d31032e

Describe the bug:

When a single error is displayed occurrence is still pluralized. I should be 1 occurrence, otherwise occurrences is fine.

Screenshots (if relevant):

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)