Fix a typo in ScopeGuard.h

Summary: Closes https://github.com/facebook/folly/pull/458

Reviewed By: yfeldblum

Differential Revision: D3765911

Pulled By: Orvid

fbshipit-source-id: 30de0b99ec622471e52a775835b5a0564437febc

folly/ScopeGuard.h

@@ -37,7 +37,8 @@ namespace folly {
  * a functor, or a void(*)() function pointer.
  *
  *
- * Usage example: Add a friend to memory iff it is also added to the db.
+ * Usage example: Add a friend to memory if and only if it is also added
+ * to the db.
  *
  * void User::addFriend(User& newFriend) {
  *   // add the friend to memory