Update premiums.md
On line 2 added "Edit and Premiums" I also capitalized Premiums as it is a UI items.
On line 3 added "Edit, Add and Premium" I also capitalized Premiums as it is a UI items.
On line 13 "Premium" capitalized.
On line 14 added space between "cost (optional) and Type (optional)".
On line 15 "Subscription and Service" capitalized.
On line 17 "Type" was capitalized to match UI.