Specify smolagents version in requirements.txt

When running offline, current code produces the following error on Python 3.12:
```bash
Traceback (most recent call last):
File "~/Agentic_RAG/app.py", line 3, in <module>
from smolagents import GradioUI, CodeAgent, HfApiModel
ImportError: cannot import name 'HfApiModel' from 'smolagents' (~/Agentic_RAG/.venv/lib/python3.12/site-packages/smolagents/__init__.py). Did you mean: 'ApiModel'?
```

The code uses `from smolagents import HfApiModel` which was removed in `smolagents 1.19`.

For simplicity, specifying `smolagents` to use `version 1.18` instead.

requirements.txt

@@ -1,5 +1,5 @@
 datasets 
-smolagents 
+smolagents==1.18.0
 langchain-community 
 rank_bm25
 duckduckgo-search