Change 'children spans' to 'child spans'

What does this MR do?

Updates the string expand children spans to expand child spans. It's a change to an ARIA label, and so invisible to users who do not rely on ARIA navigation.

In this thread we discussed various possibilities, and were nearly settled on Expand child spans. When I looked at ee/app/assets/javascripts/tracing/components/tracing_details_spans_chart.vue I realized this string probably handled both the open and closed states of the parent span, which meant expand would be wrong 50% of the time.

Toggle, then.

Related issues

Related to Review text/messages with technical writer (gitlab-org/opstrace/opstrace#2474 - closed)

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