[openssl/openssl] b2023d: doc: Fix misleading stucture info

[Ladislav Marko]

  Branch: refs/heads/master
  Home:   https://github.com/openssl/openssl
  Commit: b2023d5dfc957cf5a3cfca16961f97e79842b941
      https://github.com/openssl/openssl/commit/b2023d5dfc957cf5a3cfca16961f97e79842b941
  Author: Ladislav Marko <LadislavMarko at seznam.cz>
  Date:   2023-04-13 (Thu, 13 Apr 2023)

  Changed paths:
    M doc/man7/provider-signature.pod

  Log Message:
  -----------
  doc: Fix misleading stucture info

CLA: trivial

The thing created by `OSSL_FUNC_signature_newctx()` and `OSSL_FUNC_signature_dupctx()` is a signature context, not a signature. It's in the name of the function and surrounding documentation.

Reviewed-by: Tomas Mraz <tomas at openssl.org>
Reviewed-by: Hugo Landau <hlandau at openssl.org>
(Merged from https://github.com/openssl/openssl/pull/20705)