Update description for filter param in OpenAPI

[waltherlee]

Re: https://github.com/prometheus/alertmanager/issues/2812

filter is an array, but multiple items have to be added by repeating the param. This can be confusing since "a list of matchers" could be understood as a list separated by comma in a single value, or a group of matchers in promql format.

I'm changing the description to explicitly say that the value has to be a single matcher expression, but multiple values can be applied by repeating the param. Also fixed a typo in alert groups that said "alerts" instead of "alert groups".

The main change is in api/v2/openapi.yaml. All the others were autogenerated with make apiv2