validate: fix a typo

5aa2779461 introduced a typo.
This commit fixes it.

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit d6e28ffd27)

plugins/actions/validate.py

@@ -111,7 +111,7 @@ class ActionModule(ActionBase):
             display.error(msg)
             reason = "[{}] Reason: {}".format(host, error.reason)
             try:
-                if "schema is missing" not str(error):
+                if "schema is missing" not in str(error):
                     for i in range(0, len(error.path)):
                         if i == 0:
                             given = "[{}] Given value for {}".format(