fix comment that is outdated after PR179

Author: Carsten Griwodz

src/popsift/sift_conf.h

@@ -366,7 +366,7 @@ struct Config
     /// default: 1
     int  _filter_grid_size;
 
-    /// Modes are computation according to VLFeat or OpenCV,
+    /// Modes are computation according to VLFeat
     /// or fixed size. Default is VLFeat mode.
     GaussMode _gauss_mode;