fix concurrency docs (#1269)

need the async client
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----

> [!IMPORTANT]
> Fix import statement in `concurrent-calls.mdx` to use
`baml_client.async_client` for async operations.
> 
>   - **Documentation**:
> - In `concurrent-calls.mdx`, update import statement from
`baml_client` to `baml_client.async_client` for correct async usage in
Python example.
> 
> <sup>This description was created by </sup>[<img alt="Ellipsis"
src="https://img.shields.io/badge/Ellipsis-blue?color=175173">](https://www.ellipsis.dev?ref=BoundaryML%2Fbaml&utm_source=github&utm_medium=referral)<sup>
for 51ef0f9. It will automatically
update as commits are pushed.</sup>

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fern/01-guide/04-baml-basics/concurrent-calls.mdx

@@ -40,7 +40,7 @@ You can make concurrent `b.ClassifyMessage()` calls like so:
 ```python main.py
 import asyncio
 
-from baml_client import b
+from baml_client.async_client import b
 from baml_client.types import Category
 
 async def main():
@@ -84,4 +84,4 @@ BAML Ruby (beta) does not currently support async/concurrent calls.
 Please [contact us](/contact) if this is something you need.
 
 </Tab>
-</Tabs>
+</Tabs>