Update README.md (#1175)

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> [!IMPORTANT]
> Update image source path in `README.md`.
> 
>   - **Misc**:
> - Update image source path in `README.md` from
`docs/assets/baml-lamb-white.png` to `fern/assets/baml-lamb-white.png`.
> 
> <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 74a46ff. It will automatically
update as commits are pushed.</sup>

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README.md

@@ -1,6 +1,6 @@
 <a href="https://boundaryml.com?utm_source=github" target="_blank" rel="noopener noreferrer">
   <picture>
-    <source media="(prefers-color-scheme: dark)" srcset="docs/assets/baml-lamb-white.png">
+    <source media="(prefers-color-scheme: dark)" srcset="fern/assets/baml-lamb-white.png">
     <img src="fern/assets/baml-lamb-white.png" height="64" id="top">
   </picture>