chore: changelog entry for #9959

Author: AdrianEddy

CHANGELOG.md

@@ -80,6 +80,7 @@ Bottom level categories:
 
 - Zero-initialize padding (if any) at the end of a buffer allocation. This was application-visible in rare cases on Vulkan when a shader read beyond the valid range of a vertex buffer. By @andyleiserson in [#9791](https://github.com/gfx-rs/wgpu/pull/9791).
 - Fix required immediate slots calculation and remove `naga::valid::FunctionInfo::immediate_slots_used`. By @beicause in [#9725](https://github.com/gfx-rs/wgpu/pull/9725).
+- Fix `Buffer::unmap` failing with `NotMapped` when it races a `Buffer::map` running on another thread. By @AdrianEddy in [#9959](https://github.com/gfx-rs/wgpu/pull/9959).
 
 #### naga