Fix error message when security-updates test fails (#2568)

The error message says that `disable-targets.tests.security-updates` must be set to false if I want to disable security checks. This doesn't work. After searching around a bit, I found this comment: #1440 (comment). It seems that it should be set to true (and it makes sense, because the key name is `disable-targets`).

src/Robo/Commands/Tests/SecurityUpdatesCommand.php

@@ -21,7 +21,7 @@ public function testsSecurityUpdates() {
       ->run();
 
     if ($result->getExitCode()) {
-      $this->logger->notice('To disable security checks, set disable-targets.tests.security-updates to false in blt.yml.');
+      $this->logger->notice('To disable security checks, set disable-targets.tests.security-updates to true in blt.yml.');
       return 1;
     }
     else {