Fixed typo in spec

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%402690

dlib/threads/thread_pool_extension_abstract.h

@@ -137,7 +137,7 @@ namespace dlib
         !*/
 
         operator const T& (
-        );
+        ) const;
         /*!
             ensures
                 - if (is_ready() == false) then
@@ -159,7 +159,7 @@ namespace dlib
         !*/
 
         const T& get (
-        );
+        ) const;
         /*!
             ensures
                 - if (is_ready() == false) then