Merge pull request jupyterhub#3250 from yuvipanda/prune-2

Fix typo

Author: yuvipanda

mybinder/templates/buildkit-pruner.yaml

@@ -26,7 +26,7 @@ spec:
             - /bin/sh
             - -c
             - |
-              docker image prune ---force -all && \
+              docker image prune ---force --all && \
               docker builder prune --force --all --keep-storage={{ .Values.buildkitPruner.buildkitCacheSize }} && \
               docker system df
             volumeMounts: