stubs/colo.c: spelling

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
diff --git a/stubs/colo.c b/stubs/colo.c
index f33379d..08c9f98 100644
--- a/stubs/colo.c
+++ b/stubs/colo.c
@@ -21,7 +21,7 @@
 
 void migrate_start_colo_process(MigrationState *s)
 {
-    error_report("Impossible happend: trying to start COLO when COLO "
+    error_report("Impossible happened: trying to start COLO when COLO "
                  "module is not built in");
     abort();
 }