Fix header() invocation

robots-txt-cache.php

@@ -8,7 +8,7 @@
  */
 
 function ai_robots_txt_add_cache_headers() {
-    header("Cache-Control", "max-age=86400");
+    header("Cache-Control: max-age=86400");
 }
 
 add_action('do_robotstxt', 'ai_robots_txt_add_cache_headers');