Fix examples in Snowplow testing best practices docs

What does this MR do?

Fixes a couple of Snowplow testing examples which make use of expect_snowplow_event(…). The examples in question use a standard_context key, which as far as I can tell does not actually do anything special in regards to the Gitlab::Tracking::StandardContext. Perhaps they are from an earlier version of the Gitlab::Tracking.event method.

Does this MR meet the acceptance criteria?

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