[Store] Fix typo in TextSplitTransformer

Author: dorrogeray

src/store/src/Document/Transformer/TextSplitTransformer.php

@@ -58,7 +58,7 @@ public function __invoke(iterable $documents, array $options = []): iterable
 
                 yield new TextDocument(Uuid::v4(), $chunkText, new Metadata([
                     Metadata::KEY_PARENT_ID => $document->id,
-                    Metadata::KEY_SOURCE => $chunkText,
+                    Metadata::KEY_TEXT => $chunkText,
                     ...$document->metadata,
                 ]));