docs: Update templates README.md (langchain-ai#15013)

Mulitple -> Multiple

Author: eltociear

templates/rag-multi-index-fusion/README.md

@@ -1,4 +1,4 @@
-# RAG with Mulitple Indexes (Fusion)
+# RAG with Multiple Indexes (Fusion)
 
 A QA application that queries multiple domain-specific retrievers and selects the most relevant documents from across all retrieved results.
 
@@ -70,4 +70,4 @@ We can access the template from code with:
 from langserve.client import RemoteRunnable
 
 runnable = RemoteRunnable("http://localhost:8000/rag-multi-index-fusion")
-```
+```