fix a typo in the canny docs

doc/tutorials/imgproc/imgtrans/canny_detector/canny_detector.markdown

@@ -12,7 +12,7 @@ Theory
 ------
 
 The *Canny Edge detector* was developed by John F. Canny in 1986. Also known to many as the
-*optimal detector*, Canny algorithm aims to satisfy three main criteria:
+*optimal detector*, the Canny algorithm aims to satisfy three main criteria:
 -   **Low error rate:** Meaning a good detection of only existent edges.
 -   **Good localization:** The distance between edge pixels detected and real edge pixels have
     to be minimized.