fix typo in SharedPromise comment

Summary: ^

Test Plan: OK

Reviewed By: yfeldblum@fb.com

Subscribers: folly-diffs@, jsedgwick, yfeldblum, chalfant

FB internal diff: D2140212

Signature: t1:2140212:1433877717:86b7bf372091b455cdb25a377451e1ed4e21f50d

folly/futures/SharedPromise.h

@@ -45,8 +45,10 @@ public:
   SharedPromise(SharedPromise<T>&&) noexcept;
   SharedPromise& operator=(SharedPromise<T>&&) noexcept;
 
-  /** Return a Future tied to the shared core state. This can be called only
-    once, thereafter Future already retrieved exception will be raised. */
+  /**
+   * Return a Future tied to the shared core state. Unlike Promise::getFuture,
+   * this can be called an unlimited number of times per SharedPromise.
+   */
   Future<T> getFuture();
 
   /** Return the number of Futures associated with this SharedPromise */