rewrite attach_dir_desc

Rewrite as per https://github.com/SimpleMachines/SMF2.1/issues/288
<br/> I agree to the DCO 1.1 at https://github.com/SimpleMachines/SMF2.1/commit/b486a1e52a5f4f1b9ffae37defc6cd864926267d#DCO.txt
Corrected typo in $txt['attach_dir_desc']

I agree to the DCO 1.1 at https://github.com/SimpleMachines/SMF2.1/commit/b486a1e52a5f4f1b9ffae37defc6cd864926267d#DCO.txt
Correct Spelling in $txt['enable_disregard'] = 'Enable disregard topics';

I agree to SMF DCO statement at
https://github.com/SimpleMachines/SMF2.1/commit/b486a1e52a5f4f1b9ffae37defc6cd864926267d#DCO.txt
Merge pull request #290 from AngelinaBelle/release-2.1

Release 2.1