Fixed spelling and grammar.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403193

dlib/optimization/optimization_line_search_abstract.h

@@ -137,7 +137,7 @@ namespace dlib
               an alpha is found such that f(alpha) <= min_f then the search stops
               immediately.
             - This function is also optimized for the case where der(0) is negative.  I.e.
-              positive values of the argument to f() should be a descent direcion.  
+              positive values of the argument to f() should be in a descent direction.  
     !*/
 
     /*