Fix immutablePetCountsByEmoji in PetKata; update slides.md with solutions and corrected methods

[darshitpp]

Currently, the method immutablePetCountsByEmoji has incorrect Assertion on a Map.

Assertions.assertEquals(
        Map.of("🐱", 2L, "🐶", 2L, "🐹", 2L, "🐍", 1L, "🐢", 1L, "🐦", 1L),
        countsByEmoji
);

Here, the assertion parameter of the Map, and the refactor for countsByEmoji both need to be changed to solve the kata, which ideally should not be the case.

Additionally, the solution in slides.md itself refers to an outdated test not present in the Test Class.

Some other solutions within the same slides.md need to be refactored to have better naming.