Fix Typo (#1044)

Author: hamjo

src/Microsoft.AspNetCore.Authentication.Abstractions/AuthenticationToken.cs

@@ -5,7 +5,7 @@
 namespace Microsoft.AspNetCore.Authentication
 {
     /// <summary>
-    /// Name/Value representing an token.
+    /// Name/Value representing a token.
     /// </summary>
     public class AuthenticationToken
     {
@@ -19,4 +19,4 @@ public class AuthenticationToken
         /// </summary>
         public string Value { get; set; }
     }
-}
+}