Fix another comment

Summary:
I don't know what this paragraph is even saying. It looks like
copypasta

Test Plan: still compiles or my uncle is a monkey

Reviewed By: davejwatson@fb.com

Subscribers: trunkagent, exa, folly-diffs@, jsedgwick, yfeldblum

FB internal diff: D1819905

Tasks: 5705286

Signature: t1:1819905:1423074021:a095fe2a17d5698678a527b60c9384625df869fd

folly/futures/Future.h

@@ -199,10 +199,6 @@ class Future {
   /** Return the reference to result. Should not be called if !isReady().
     Will rethrow the exception if an exception has been
     captured.
-
-    This function is not thread safe - the returned Future can only
-    be executed from the thread that the executor runs it in.
-    See below for a thread safe version
     */
   typename std::add_lvalue_reference<T>::type
   value();