Fix 'backwards' typo

[ttous]

Sources:

• https://wordstylehq.com/backward-vs-backwards
• https://www.quickanddirtytips.com/articles/backward-versus-backwards/
• https://writingexplained.org/backward-or-backwards-difference

[ljharb]

I'd hardly call this a "typo" since i've never heard "backwards compatible" causing confusion, and since language is descriptive and not proscriptive, that means it's correct as-is.

That said, this doesn't seem harmful.

[mbtools]

Agree with the author. It should be "backward"

[JohnTitor]

All the use of "backwards" has "compatible" or "compatibility" and I believe "backwards" is also valid in that context (as Wikipedia says).
I don't have a strong reason to reject though, I think we could merge without RFC as it doesn't have any semantic changes.