adding SentenceSplitter to init imports (#8644)

haystack/components/preprocessors/__init__.py

@@ -5,6 +5,7 @@
 from .document_cleaner import DocumentCleaner
 from .document_splitter import DocumentSplitter
 from .nltk_document_splitter import NLTKDocumentSplitter
+from .sentence_tokenizer import SentenceSplitter
 from .text_cleaner import TextCleaner
 
-__all__ = ["DocumentSplitter", "DocumentCleaner", "TextCleaner", "NLTKDocumentSplitter"]
+__all__ = ["DocumentSplitter", "DocumentCleaner", "NLTKDocumentSplitter", "SentenceSplitter", "TextCleaner"]