qcow2: A grammar fix in conflicting cache sizing error message

Signed-off-by: Leonid Bloch <lbloch@janustech.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/block/qcow2.c b/block/qcow2.c
index 6162ed8..ec9e623 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -797,7 +797,7 @@
         if (l2_cache_size_set && refcount_cache_size_set) {
             error_setg(errp, QCOW2_OPT_CACHE_SIZE ", " QCOW2_OPT_L2_CACHE_SIZE
                        " and " QCOW2_OPT_REFCOUNT_CACHE_SIZE " may not be set "
-                       "the same time");
+                       "at the same time");
             return;
         } else if (*l2_cache_size > combined_cache_size) {
             error_setg(errp, QCOW2_OPT_L2_CACHE_SIZE " may not exceed "