Fixed typo in spec.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403012

dlib/sliding_buffer/sliding_buffer_kernel_abstract.h

@@ -83,9 +83,9 @@ namespace dlib
         );
         /*!
             requires
-                - 0 < expsize < 32
+                - 0 < exp_size < 32
             ensures
-                - #size() == 2^expsize
+                - #size() == 2^exp_size
                 - the value of all elements in the buffer are undefined
                 - #at_start() == true
             throws