Update hosting.rst -- Correct "on-premises" misspellings

[cparker4486]

Hello Odoo team!

"premise" and "premises" are completely different words. Premise is a proposition and premises is a building or location.

"on-premise" doesn't mean anything. The term you want to be using when referring to the concept of self-hosting is "on-premises". This is a very common mistake, and reasonably so, but it should be corrected.

The phrase "on-premise" is used throughout Odoo's marketing and documentation but I'm only able to offer this one pull request.

[robodoo]

Pull request status dashboard.