crypto: doc - Fix akcipher title reference

Stephen reports a documentation build warning for "make htmldocs"
introduced by recent commit 6b34562f ("crypto: akcipher - Drop
sign/verify operations").

The commit renamed a paragraph title in a header file, but neglected to
amend the title reference in the API documentation.  Fix it.

Fixes: 6b34562f ("crypto: akcipher - Drop sign/verify operations")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/all/20241008172926.0b995ea7@canb.auug.org.au/


Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>