update generic extends example

pages/Generics.md

@@ -269,21 +269,18 @@ loggingIdentity({length: 10, value: 3});
 ## Using Type Parameters in Generic Constraints
 
 You can declare a type parameter that is constrained by another type parameter.
-For example, here we'd like to take two objects and copy properties from one to the other.
-We'd like to ensure that we're not accidentally writing any extra properties from our `source`, so we'll place a constraint between the two types:
+For example, here we'd like to get a property from an object given its name.
+We'd like to ensure that we're not accidentally grabbing a property that does not exist on the `obj`, so we'll place a constraint between the two types:
 
 ```ts
-function copyFields<T extends U, U>(target: T, source: U): T {
-    for (let id in source) {
-        target[id] = source[id];
-    }
-    return target;
+function getProperty<T, K extends keyof T>(obj: T, key: K) {
+    return obj[key];
 }
 
 let x = { a: 1, b: 2, c: 3, d: 4 };
 
-copyFields(x, { b: 10, d: 20 }); // okay
-copyFields(x, { Q: 90 });  // error: property 'Q' isn't declared in 'x'.
+getProperty(x, "a"); // okay
+getProperty(x, "m"); // error: Argument of type 'm' isn't assignable to 'a' | 'b' | 'c' | 'd'.
 ```
 
 ## Using Class Types in Generics