Fix a nameof() using wrong variable in GeneratedVariableIdentifier

src/Generator.Rewrite/GeneratedVariableIdentifier.cs

@@ -40,7 +40,7 @@ namespace OpenTK.Rewrite
 
             if (definition == null)
             {
-                throw new ArgumentException("The definition argument cannot be null.", nameof(body));
+                throw new ArgumentException("The definition argument cannot be null.", nameof(definition));
             }
 
             if (string.IsNullOrEmpty(name))
@@ -53,4 +53,4 @@ namespace OpenTK.Rewrite
             this.Name = name;
         }
     }
-}
+}