Update single.yml

fix, replace [space] with _
diff --git a/ceph/client/single.yml b/ceph/client/single.yml
index 7c0bd63..6ff7cd8 100644
--- a/ceph/client/single.yml
+++ b/ceph/client/single.yml
@@ -21,10 +21,10 @@
         mon:
           mon_debug_dump_transactions: False
         client:
-          rbd cache size: 268435456
-          rbd cache max dirty: 134217728
-          rbd cache max dirty age: 5
-          rbd cache: True
+          rbd_cache_size: 268435456
+          rbd_cache_max_dirty: 134217728
+          rbd_cache_max_dirty_age: 5
+          rbd_cache: True
       # TODO: Configure these keys on cluster level
       # keyring:
       #   images: