Inconsistency in spelling of "dataset"

[dansysanalyst]

Q	A
Bug fix?	no
New feature?	no
Fixed tickets

This PR proposes to alter "data set" to "dataset" in test descriptions.

It seems the most often used spelling is "dataset".

However, in the tests description and in an Exception, it is spelled "data set".

Below, we have both cases in one sentence.

Consistent usage

Documentation: https://pestphp.com/docs/datasets

Docblock:

    /**
     * Ensures dataset items count matches underlying test case required parameters
     * ...
     */