Clarify file creation instruction in Quickstart section of README.md (#11089)

## Describe your changes

Updated the "Quickstart" section in README.md to clarify where to create
the file for beginners.
Changed "Create a new file streamlit_app.py with the following code:" to
"Create a new file named `streamlit_app.py` in your project directory
with the following code:".
No issue was created as this is a small documentation fix, per the
Contributing Uncyclo
(https://github.com/streamlit/streamlit/wiki/Contributing).

## GitHub Issue Link (if applicable)

N/A

## Testing Plan

- **Explanation of why no additional tests are needed**: This is a
documentation change in README.md, which does not affect the codebase or
functionality of Streamlit. As per the Contributing Uncyclo, tests are not
required for documentation updates.
- **Unit Tests (JS and/or Python)**: Not applicable.
- **E2E Tests**: Not applicable.
- **Any manual testing needed?** I previewed the updated README.md using
VS Code’s Markdown Preview to confirm the change renders correctly and
improves clarity for beginners.

---

**Contribution License Agreement**

By submitting this pull request you agree that all contributions to this
project are made under the Apache 2.0 license.

Author: Ahmednoorani258

README.md

@@ -36,7 +36,7 @@ The app features a bunch of examples of what you can do with Streamlit. Jump to
 
 ### A little example
 
-Create a new file `streamlit_app.py` with the following code:
+Create a new file named `streamlit_app.py` in your project directory with the following code:
 ```python
 import streamlit as st
 x = st.slider("Select a value")