Fixed incorrect docs

docs/docs/imaging.xml

@@ -843,8 +843,8 @@
          <file>dlib/image_transforms.h</file>
          <spec_file link="true">dlib/image_transforms/thresholding_abstract.h</spec_file>
          <description>        
-            This global function performs a simple binary thresholding on an image with a user
-            supplied threshold.  However, instead of taking a user supplied threshold
+            This global function performs a simple binary thresholding on an image.  
+            Instead of taking a user supplied threshold
             it computes one from the image using k-means clustering.
          </description>