MM-17492 Update error message for CustomURLSchemes setting (#11895)

i18n/en.json

@@ -4464,7 +4464,7 @@
   },
   {
     "id": "model.config.is_valid.display.custom_url_schemes.app_error",
-    "translation": "The custom URL scheme {{.Scheme}} is invalid. Custom URL schemes must start with a letter and contain only letters, numbers and hyphen (-)."
+    "translation": "The custom URL scheme {{.Scheme}} is invalid. Custom URL schemes must start with a letter and contain only letters, numbers, plus (+), period (.) and hyphen (-)."
   },
   {
     "id": "model.config.is_valid.elastic_search.aggregate_posts_after_days.app_error",