Made spec a little more clear.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403575

dlib/statistics/statistics_abstract.h

@@ -153,7 +153,7 @@ namespace dlib
             requires
                 - current_n() > 1
             ensures
-                - return (val-mean())/std::sqrt(variance());
+                - return (val-mean())/stddev();
         !*/
     };