Update PhoneNumberTokenProvider.cs

Add license header.
Hyphenate two-factor authentication in documentation.
Add line breaks before return statements.
Use string interpolation instead of string concatenation.

src/Identity/Extensions.Core/src/PhoneNumberTokenProvider.cs

@@ -1,3 +1,6 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
 using System;
 using System.Threading.Tasks;
 
@@ -12,7 +15,7 @@ namespace Microsoft.AspNetCore.Identity
         where TUser : class
     {
         /// <summary>
-        /// Returns a flag indicating whether the token provider can generate a token suitable for two factor authentication token for
+        /// Returns a flag indicating whether the token provider can generate a token suitable for two-factor authentication token for
         /// the specified <paramref name="user"/>.
         /// </summary>
         /// <param name="manager">The <see cref="UserManager{TUser}"/> that can be used to retrieve user properties.</param>
@@ -20,7 +23,7 @@ namespace Microsoft.AspNetCore.Identity
         /// <returns>
         /// The <see cref="Task"/> that represents the asynchronous operation, containing the a flag indicating if a two
         /// factor token could be generated by this provider for the specified <paramref name="user"/>.
-        /// The task will return true if a two factor authentication token could be generated as the user has
+        /// The task will return true if a two-factor authentication token could be generated as the user has
         /// a telephone number, otherwise false.
         /// </returns>
         public override async Task<bool> CanGenerateTwoFactorTokenAsync(UserManager<TUser> manager, TUser user)
@@ -29,7 +32,9 @@ namespace Microsoft.AspNetCore.Identity
             {
                 throw new ArgumentNullException(nameof(manager));
             }
+
             var phoneNumber = await manager.GetPhoneNumberAsync(user);
+
             return !string.IsNullOrWhiteSpace(phoneNumber) && await manager.IsPhoneNumberConfirmedAsync(user);
         }
 
@@ -49,8 +54,10 @@ namespace Microsoft.AspNetCore.Identity
             {
                 throw new ArgumentNullException(nameof(manager));
             }
+
             var phoneNumber = await manager.GetPhoneNumberAsync(user);
-            return "PhoneNumber:" + purpose + ":" + phoneNumber;
+
+            return $"{PhoneNumber}:{purpose}:{phoneNumber}";
         }
     }
-}
+}