go-ceph: fix typo in doc.go
It's 'rbd' not 'rdb'.
@@ -6,7 +6,7 @@ and contains no functions.
The "rados" sub-package wraps APIs that handle RADOS specific functions.
-The "rdb" sub-package wraps APIs that handle RBD specific functions.
+The "rbd" sub-package wraps APIs that handle RBD specific functions.
The "cephfs" sub-package wraps APIs that handle CephFS specific functions.
