Correct exercise solutions

[3kori]

Closes #255

This PR adds missing characters to some of the solutions, making them correct per the question.

1. In "How would you find a 4-letter word that ends a string and is preceded by at least one zero?" the challenge expects any 4-letter word like "Four", "four", "FOUR", "fOuR", etc. But the current solution limits the 4-letter word to lowercase letters only.

0+[a-z]{4}\b instead of 0+[A-Za-z]{4}\b

2. In "How would you match publication formats such as British Library : London, 2015 and Manchester University Press: Manchester, 1999?" the answer only matches the first format. It doesn't match the other format, which doesn't have a space just before the colon (:).

.* : .*, \d{4} instead of .* ?: .*, \d{4}

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