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

Add missing underscore.

Signed-off-by: Baruch Siach <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/crypto/async-tx-api.rst

···
 ::
 
   struct dma_async_tx_descriptor *
-  async_<operation>(<op specific parameters>, struct async_submit ctl *submit)
+  async_<operation>(<op specific parameters>, struct async_submit_ctl *submit)
 
 3.2 Supported operations
 ------------------------