Alias Book_cover -> Cover_Page

Alias Book_Cover -> Cover_Page

Reason: I cleaned up the tags in book cover and changed them to cover page, for consistency since there weren't that many for book cover.

I feel it should be aliased since the wiki for cover pages includes book covers.

Done, although someone might be able to argue for book cover as a separate tag, in which case I'll remove the alias.

I felt that it could be placed as a separate tag but couldn't cover page just be slightly amended to include book covers?