update spec reference

Author: FynnBe

README.md

@@ -366,7 +366,7 @@ may be controlled with the `LOGURU_LEVEL` environment variable.
 
 ### 0.9.0 (coming soon)
 
-- update to [bioimageio.spec 0.5.4.2](https://github.com/bioimage-io/spec-bioimage-io/blob/main/changelog.md#bioimageiospec-0542)
+- update to [bioimageio.spec 0.5.4.3](https://github.com/bioimage-io/spec-bioimage-io/blob/main/changelog.md#bioimageiospec-0543)
 
 ### 0.8.0