Fix P-521 key parameter size
Fix P-521 key parameter size

The points on P-521 are 66 bytes, not 64. This is likely a leftover
from being incorrectly referred to as P-512.

Fixes: https://github.com/AzureAD/azure-activedirectory-identitymodel-extensions-for-dotnet/issues/882

AzureAD/azure-activedirectory-identitymodel-extensions-for-dotnet,AzureAD/azure-activedirectory-identitymodel-extensions-for-dotnet,AzureAD/azure-activedirectory-identitymodel-extensions-for-dotnet,AzureAD/azure-activedirectory-identitymodel-extensions-for-dotnet