crypto: ecdsa - Update Kconfig help text for NIST P521

Commit a7d45ba7 ("crypto: ecdsa - Register NIST P521 and extend test
suite") added support for ECDSA signature verification using NIST P521,
but forgot to amend the Kconfig help text.  Fix it.

Fixes: a7d45ba7 ("crypto: ecdsa - Register NIST P521 and extend test suite")
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>