Fix type name typo in d2i/i2d documentation.

Reviewed-by: NMatt Caswell <matt@openssl.org>
Reviewed-by: NShane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/10933)

doc/man3/d2i_X509.pod

@@ -491,7 +491,7 @@ Represents an ASN1 OBJECT IDENTIFIER.
 
 Represents a PKCS#3 DH parameters structure.
 
-=item B<DHparamx>
+=item B<DHxparams>
 
 Represents an ANSI X9.42 DH parameters structure.