fix ExceptionWrapper comment

Summary:
Fix comment for exception_wrapper's `operator bool()`.
It reads "return `true` if `*this` is not holding an exception"
but the actual behavior is to return `true` if `*this` *is* holding
an exception.

Reviewed By: yfeldblum

Differential Revision: D5456386

fbshipit-source-id: ec3a472ea441476d835881e94036a3957bb2ab32

folly/ExceptionWrapper.h

@@ -444,7 +444,7 @@ class exception_wrapper final {
   //! Swaps the value of `*this` with the value of `that`
   void swap(exception_wrapper& that) noexcept;
 
-  //! \return `true` if `*this` is not holding an exception.
+  //! \return `true` if `*this` is holding an exception.
   explicit operator bool() const noexcept;
 
   //! \return `!bool(*this)`