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>

Author: baruchsiach

Documentation/crypto/async-tx-api.rst

@@ -66,7 +66,7 @@ features surfaced as a result:
 ::
 
   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
 ------------------------