Language: replace Each x hours with Every x hours

Hi,

I just started using Tiny-Tiny-RSS - thanks!

One thing that I noticed was that the language "each x hours" doesn't really seem right. As a native English speaker, we would always say "every x hours", never "each x hours". Perhaps sometimes we might say "each and every x hours", if we really wanted to add emphasis, but usually just "every" is fine.

So, this pull request changes "each" to "every". That's it.

Thanks!