fix: use latest version of ml_dtypes in example script

Author: d-v-b

examples/custom_dtype/custom_dtype.py

@@ -2,7 +2,7 @@
 # requires-python = ">=3.12"
 # dependencies = [
 #   "zarr @ git+https://github.com/zarr-developers/zarr-python.git@main",
-#   "ml_dtypes==0.5.1",
+#   "ml_dtypes==0.5.4",
 #   "pytest==8.4.1"
 # ]
 # ///