Fixed spelling error.

dlib/image_processing/scan_image_pyramid_abstract.h

@@ -65,7 +65,7 @@ namespace dlib
                           rectangle.
                         - Then the feature vector for a sliding window is an M*N dimensional vector
                           [F(1) F(2) F(3) ... F(N)] (i.e. it is a concatenation of the N vectors).
-                          This feature vector can be though of as a collection of N "bags of features",
+                          This feature vector can be thought of as a collection of N "bags of features",
                           each bag coming from a spatial location determined one of the enveloping 
                           rectangles.