docs: fix typos in Kandinsky 5.0 video docs (#14345)

- `qualty` -> `quality`
- `infered` -> `inferred` (x3)
- `bakend` -> `backend` (x3, in the trailing `# <--- Set attention
  bakend to Flex` comments; the actual `set_attention_backend` calls
  were already spelled correctly)
- `wothout` -> `without`

Prose and comments only, no code or behavior changes.