Fixed spelling issue.

Author: Jesper Glintborg

src/NetMQ.Tests/NetMQCertificateTest.cs

@@ -12,10 +12,10 @@ public void X85()
             for (int i = 0; i < 1000; i++)
             {
                 var key = new NetMQCertificate();
-                var copy  = new NetMQCertificate(key.SecretKeyX85, key.PublicKeyX85);
+                var copy  = new NetMQCertificate(key.SecretKeyZ85, key.PublicKeyZ85);
 
-                Assert.Equal(key.SecretKeyX85, copy.SecretKeyX85);
-                Assert.Equal(key.PublicKeyX85, copy.PublicKeyX85);
+                Assert.Equal(key.SecretKeyZ85, copy.SecretKeyZ85);
+                Assert.Equal(key.PublicKeyZ85, copy.PublicKeyZ85);
 
                 Assert.Equal(key.SecretKey, copy.SecretKey);
                 Assert.Equal(key.PublicKey, copy.PublicKey);

src/NetMQ/NetMQCertificate.cs

@@ -247,7 +247,7 @@ public static NetMQCertificate FromPublicKey(string publicKey)
         /// <summary>
         /// Curve Public key 
         /// </summary>
-        public string SecretKeyX85 => SecretKey != null ? Z85Encode(SecretKey) : null;
+        public string SecretKeyZ85 => SecretKey != null ? Z85Encode(SecretKey) : null;
 
 
         /// <summary>
@@ -263,7 +263,7 @@ public static NetMQCertificate FromPublicKey(string publicKey)
         /// <summary>
         /// Curve Public key 
         /// </summary>
-        public string PublicKeyX85 
+        public string PublicKeyZ85 
         {
             get => Z85Encode(PublicKey);
         }