Fix GLEX testing examples: on_any_instance => on_next_instance

What does this MR do?

Fixes a couple of examples in the gitlab-experiment (GLEX) docs which show how to test for experiment-specific tracked events. The examples make use of on_any_instance, but I believe that that method has been removed from our codebase and the recommended approach is to use on_next_instance instead.

Does this MR meet the acceptance criteria?

Conformity

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