Order the definitions to match the order of terms (#4576)

The introductory sentence could be read as implying
definitions that contradict the body of the article.

docs/standard/generics/covariance-and-contravariance.md

@@ -26,7 +26,7 @@ ms.workload:
   - "dotnetcore"
 ---
 # Covariance and Contravariance in Generics
-<a name="top"></a> Covariance and contravariance are terms that refer to the ability to use a less derived (less specific) or more derived type (more specific) than originally specified. Generic type parameters support covariance and contravariance to provide greater flexibility in assigning and using generic types. When you are referring to a type system, covariance, contravariance, and invariance have the following definitions. The examples assume a base class named `Base` and a derived class named `Derived`.  
+<a name="top"></a> Covariance and contravariance are terms that refer to the ability to use a more derived type (more specific) or a less derived type (less specific) than originally specified. Generic type parameters support covariance and contravariance to provide greater flexibility in assigning and using generic types. When you are referring to a type system, covariance, contravariance, and invariance have the following definitions. The examples assume a base class named `Base` and a derived class named `Derived`.  
 
 -   `Covariance`