Update README date

Author: olupton

README.md

@@ -230,7 +230,7 @@ For a list of previously used XLA flags that are no longer needed, please also r
 
 | First nightly with new base container | Base container |
 | ------------------------------------- | -------------- |
-| 2025-05-?? | nvcr.io/nvidia/cuda-dl-base:25.05-cuda12.9-devel-ubuntu24.04 |
+| 2025-05-31 | nvcr.io/nvidia/cuda-dl-base:25.05-cuda12.9-devel-ubuntu24.04 |
 | 2025-04-11 | nvcr.io/nvidia/cuda-dl-base:25.03-cuda12.8-devel-ubuntu24.04 |
 | 2025-03-04 | nvcr.io/nvidia/cuda-dl-base:25.02-cuda12.8-devel-ubuntu24.04 |
 | 2025-01-31 | nvcr.io/nvidia/cuda-dl-base:25.01-cuda12.8-devel-ubuntu24.04 |