Fix regex in `Document.get_sentences()`

            books[document.book_id].append(document)

        def get_sentences(text: str) -> Iterable[Sentence]:

            sentence_regex = (

                r'.*\P{L}\p{Ll}+\.\P{L}+(?P<sentence>\p{Lu}\p{Ll}*\P{L}.*?\P{L}\p{Ll}+\.)\P{L}+\p{Lu}\p{Ll}*(?:\P{L}|$)'

            )

            prefix_regex = r'\P{L}\p{Ll}+\.\P{L}+'

            match_regex = r'((?P<sentence_head>\p{Lu}\p{Ll}*\P{L}.*?)'

            suffix_regex = r'(?=(?P<sentence_tail>\P{L}\p{Ll}+\.)\P{L}+\p{Lu}\p{Ll}*(?:\P{L}|$)))'

            sentence_regex = f'{prefix_regex}{match_regex}{suffix_regex}'

            for match in regex.finditer(sentence_regex, text):

                sentence = match.group('sentence')

                sentence_head = match.group('sentence_head')

                sentence_tail = match.group('sentence_tail')

                sentence = f'{sentence_head}{sentence_tail}'

                sentence = regex.sub(r'\n+', ' ', sentence)

                yield sentence