Fix folly::Singleton error message

Test Plan: fbmake runtests

Reviewed By: chip@fb.com, pavlo@fb.com

Subscribers: folly-diffs@, yfeldblum

FB internal diff: D1825369

Signature: t1:1825369:1423080185:8507643a3f8860bae6a2240e0ae28fbaf2885b12

folly/experimental/Singleton.cpp

@@ -73,7 +73,8 @@ void SingletonVault::destroyInstance(SingletonMap::iterator entry_it) {
                << "reference at destroyInstances time; beware! Raw pointer "
                << "is " << entry.instance_ptr << ". It is very likely that "
                << "some other singleton is holding a shared_ptr to it. Make "
-               << "dependencies between these singletons are properly defined.";
+               << "sure dependencies between these singletons are properly "
+               << "defined.";
   }
 }