[docs] slight edits to the attention backends docs. (#12394)
* slight edits to the attention backends docs.
* Update docs/source/en/optimization/attention_backends.md
Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>
---------
Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>