signal handler: use mariadb kb URL rather than MySQL one

sql/signal_handler.cc

@@ -229,7 +229,7 @@ extern "C" sig_handler handle_fatal_signal(int sig)
   }
   my_safe_printf_stderr("%s",
     "The manual page at "
-    "http://dev.mysql.com/doc/mysql/en/crashing.html contains\n"
+    "https://mariadb.com/kb/en/how-to-produce-a-full-stack-trace-for-mysqld/ contains\n"
     "information that should help you find out what is causing the crash.\n");
 
 #endif /* HAVE_STACKTRACE */