If your goal is to transfer all files from local_dir the * wildcard does the trick:
$ scp ~/local_dir/* [email protected]:/var/www/html/target_dir
The -r option means "recursively", so you must write it when you're trying to transfer an entire directory or several directories.
From man scp:
-r
Recursively copy entire directories. Note that scp follows symbolic links encountered in the tree traversal.
So if you have sub-directories inside local_dir, the last example will only transfer files, but if you set the -r option, it will transfer files and directories.