storage: fix typo in previous patch
    
    You'd think I'd learn to actually COMMIT my working tree
    between testing that a last-minute fix compiles and pushing.
    
    * src/storage/storage_backend_gluster.c
    (virStorageBackendGlusterRefreshVol): Typo fix.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>