tutorial typo fix (#311)

DocumenStore -> DocumentStore

Author: absnormal

tutorials/27_First_RAG_Pipeline.ipynb

@@ -207,9 +207,9 @@
       "source": [
         "## Fetching and Indexing Documents\n",
         "\n",
-        "You'll start creating your question answering system by downloading the data and indexing the data with its embeddings to a DocumenStore. \n",
+        "You'll start creating your question answering system by downloading the data and indexing the data with its embeddings to a DocumentStore. \n",
         "\n",
-        "In this tutorial, you will take a simple approach to writing documents and their embeddings into the DocumenStore. For a full indexing pipeline with preprocessing, cleaning and splitting, check out our tutorial on [Preprocessing Different File Types](https://haystack.deepset.ai/tutorials/30_file_type_preprocessing_index_pipeline).\n",
+        "In this tutorial, you will take a simple approach to writing documents and their embeddings into the DocumentStore. For a full indexing pipeline with preprocessing, cleaning and splitting, check out our tutorial on [Preprocessing Different File Types](https://haystack.deepset.ai/tutorials/30_file_type_preprocessing_index_pipeline).\n",
         "\n",
         "\n",
         "### Initializing the DocumentStore\n",