Docs: Fix typos in question_answering (#15565)

**Description**: Fixed a minor typo in the RAG Docs:
- ~~This usually happen offline~~ -> This usually happen**s** offline

docs/docs/use_cases/question_answering/index.ipynb

@@ -50,7 +50,7 @@
     "## RAG Architecture\n",
     "A typical RAG application has two main components:\n",
     "\n",
-    "**Indexing**: a pipeline for ingesting data from a source and indexing it. *This usually happen offline.*\n",
+    "**Indexing**: a pipeline for ingesting data from a source and indexing it. *This usually happens offline.*\n",
     "\n",
     "**Retrieval and generation**: the actual RAG chain, which takes the user query at run time and retrieves the relevant data from the index, then passes that to the model.\n",
     "\n",

docs/docs/use_cases/question_answering/quickstart.ipynb

@@ -42,7 +42,7 @@
     "## Architecture\n",
     "We'll create a typical RAG application as outlined in the [Q&A introduction](/docs/use_cases/question_answering/), which has two main components:\n",
     "\n",
-    "**Indexing**: a pipeline for ingesting data from a source and indexing it. *This usually happen offline.*\n",
+    "**Indexing**: a pipeline for ingesting data from a source and indexing it. *This usually happens offline.*\n",
     "\n",
     "**Retrieval and generation**: the actual RAG chain, which takes the user query at run time and retrieves the relevant data from the index, then passes that to the model.\n",
     "\n",