fix a message

resource/locales/en-US/translation.json

@@ -322,7 +322,7 @@
     "sitename_change": "You can change Site Name which is used for header and HTML title.",
     "header_content": "The contents entered here will be shown in the header etc.",
     "Site URL desc": "This is for the site URL setting.",
-    "Site URL warn": "Some features don't work because site URL is not set.",
+    "Site URL warn": "Some features don't work because the site URL is not set.",
     "siteurl_help": "Site full URL beginning from <code>http://</code> or <code>https://</code>.",
     "Confidential name": "Confidential name",
     "Default Language for new users": "Default Language for new users",