docs: updated document for 'Return Source Documents' Functionality (#15106)

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- **Description:** updated the outdated code in the document that was
generating the error,
  - **Issue:** #15086 ,
  - **Dependencies:** N/A,
- **Twitter handle:** [@vardhaman722](https://twitter.com/vardhaman722)

docs/docs/integrations/providers/vectara/vectara_chat.ipynb

@@ -192,7 +192,7 @@
    "outputs": [],
    "source": [
     "query = \"What did the president say about Ketanji Brown Jackson\"\n",
-    "result = bot({\"question\": query})"
+    "result = bot.invoke({\"question\": query})"
    ]
   },
   {
@@ -224,7 +224,7 @@
    "outputs": [],
    "source": [
     "query = \"Did he mention who she suceeded\"\n",
-    "result = bot({\"question\": query})"
+    "result = bot.invoke({\"question\": query})"
    ]
   },
   {
@@ -291,7 +291,7 @@
    "source": [
     "chat_history = []\n",
     "query = \"What did the president say about Ketanji Brown Jackson\"\n",
-    "result = bot({\"question\": query, \"chat_history\": chat_history})"
+    "result = bot.invoke({\"question\": query, \"chat_history\": chat_history})"
    ]
   },
   {
@@ -336,7 +336,7 @@
    "source": [
     "chat_history = [(query, result[\"answer\"])]\n",
     "query = \"Did he mention who she suceeded\"\n",
-    "result = bot({\"question\": query, \"chat_history\": chat_history})"
+    "result = bot.invoke({\"question\": query, \"chat_history\": chat_history})"
    ]
   },
   {
@@ -396,7 +396,7 @@
    "source": [
     "chat_history = []\n",
     "query = \"What did the president say about Ketanji Brown Jackson\"\n",
-    "result = bot({\"question\": query, \"chat_history\": chat_history})"
+    "result = bot.invoke({\"question\": query, \"chat_history\": chat_history})"
    ]
   },
   {
@@ -646,7 +646,7 @@
    "source": [
     "chat_history = []\n",
     "query = \"What did the president say about Ketanji Brown Jackson\"\n",
-    "result = bot({\"question\": query, \"chat_history\": chat_history})"
+    "result = bot.invoke({\"question\": query, \"chat_history\": chat_history})"
    ]
   },
   {
@@ -668,7 +668,7 @@
    "source": [
     "chat_history = [(query, result[\"answer\"])]\n",
     "query = \"Did he mention who she suceeded\"\n",
-    "result = bot({\"question\": query, \"chat_history\": chat_history})"
+    "result = bot.invoke({\"question\": query, \"chat_history\": chat_history})"
    ]
   },
   {
@@ -712,7 +712,7 @@
    "source": [
     "chat_history = []\n",
     "query = \"What did the president say about Ketanji Brown Jackson\"\n",
-    "result = bot({\"question\": query, \"chat_history\": chat_history})"
+    "result = bot.invoke({\"question\": query, \"chat_history\": chat_history})"
    ]
   },
   {