fix typo in DogHairLenghtComparer

typo in Equals when validating objects on null

Author: AlexTroshkin

docs/concepts/linq.md

@@ -150,7 +150,7 @@ public class DogHairLengthComparer : IEqualityComparer<Dog>
 {
     public bool Equals(Dog a, Dog b)
     {
-        if (a == null && a == null)
+        if (a == null && b == null)
         {
             return true;
         }
@@ -256,4 +256,4 @@ Parallelizable CPU-bound jobs which can be easily expressed via LINQ (in other w
 
 *   [101 LINQ Samples](https://code.msdn.microsoft.com/101-LINQ-Samples-3fb9811b)
 *   [Linqpad](https://www.linqpad.net/), a playground environment and Database querying engine for C#/F#/VB
-*   [EduLinq](http://codeblog.jonskeet.uk/2011/02/23/reimplementing-linq-to-objects-part-45-conclusion-and-list-of-posts/), an e-book for learning how LINQ-to-objects is implemented
+*   [EduLinq](http://codeblog.jonskeet.uk/2011/02/23/reimplementing-linq-to-objects-part-45-conclusion-and-list-of-posts/), an e-book for learning how LINQ-to-objects is implemented