Minor fixes in Futures.md (#911)

Summary:
lvalue-qualified then() has been deprecated
lvalue-qualified get() has been deleted
Pull Request resolved: https://github.com/facebook/folly/pull/911

Reviewed By: LeeHowes

Differential Revision: D9310836

Pulled By: yfeldblum

fbshipit-source-id: 662596bf5985f7ceabb2a23725d51f8e05407340

folly/docs/Futures.md

@@ -26,14 +26,14 @@ void foo(int x) {
   cout << "making Promise" << endl;
   Promise<int> p;
   Future<int> f = p.getSemiFuture().via(&executor);
-  auto f2 = f.then(foo);
+  auto f2 = move(f).then(foo);
   cout << "Future chain made" << endl;
 
 // ... now perhaps in another event callback
 
   cout << "fulfilling Promise" << endl;
   p.setValue(42);
-  f2.get();
+  move(f2).get();
   cout << "Promise fulfilled" << endl;
 ```