Fixes a trivial bug in Idenitity.cs found by code scan

Author: J Wyman

LibGit2Sharp/Identity.cs

@@ -20,7 +20,7 @@ public Identity(string name, string email)
             Ensure.ArgumentNotNullOrEmptyString(name, "name");
             Ensure.ArgumentNotNullOrEmptyString(email, "email");
             Ensure.ArgumentDoesNotContainZeroByte(name, "name");
-            Ensure.ArgumentDoesNotContainZeroByte(name, "email");
+            Ensure.ArgumentDoesNotContainZeroByte(email, "email");
 
             _name = name;
             _email = email;