Aggregation: Fix copy of missing groups tooltip

Created by: vovakulikov

Follow up from https://github.com/sourcegraph/sourcegraph/pull/41212#discussion_r960889857

We need to update copy. Suggested copy is

There are ${missingCount} more groups that are not shown.

Also fix the copy pluralization

we need to handle pluralization for the case of missingCount being 1: In that case, it needs to say "There is 1 more group that is not shown."

/cc @joelkw @felixfbecker @vovakulikov