docs: crypto: async-tx-api: fix typo in struct name

Add missing underscore.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Link: https://lore.kernel.org/r/2ef9dfaa33c1eff019e6fe43fe738700c2230b3d.1685342291.git.baruch@tkos.co.il


Signed-off-by: Jonathan Corbet <corbet@lwn.net>