Rename 'is_dist' to 'isdst' in locale.c documentation

[tobyink]

I assume the use of "is_dist" was a typo.

I haven't included a perldelta entry as it is just a small documentation clarification, but let me know if you think it needs one.

[khwilliamson]

Thank you for this. It can't be merged until your email address is updated in our records. I think the way to do that most easily is to run Porting/updateAUTHORS.pl and then commit the changes. Ideally you would then swap the order of the commits so that your main commit wouldn't fail porting tests, but most people don't bother.