[docs] Fix typo for some words at 0001-calendar-improvements.md

[lamtrinhdev]

Fix typo for some words at 0001-calendar-improvements.md

• manully => manually
• discrepency => discrepancy
• diffcult => difficult

[lamtrinhdev]

Dear @itingliu , @parkera ,

I have a minor update at 0001-calendar-improvements.md with correct the word. Could you please help me to review it?

Thanks,
Lam

[parkera]

Thanks! Running CI for merge purposes

[parkera]

@swift-ci test

[lamtrinhdev]

Dear @parkera ,

Thank you for your review and support. I do not possess the authority to merge. When you have time, could you please help me to merge it?

Thanks,
Lam