Inconsistency: aggregating vs aggregate

The sum and count Kafka tutorials instruct the user to create a directory called `aggregate-`, whereas the actual folder for them in GitHub are called `aggregating-`.

There is no functional impact to current behavior (it still works) but it would be better to be consistent.

Compare the directory instructions in the sum and count Kafka tutorials with their corresponding GitHub folders. Start by locating the references to `aggregate-` and `aggregating-`; done means the tutorials consistently use the actual directory name, with no functional behavior changed.

Written by the indexing model from the issue text.