fix typo: valoe -> value (#833)

docs/generating_the_url.md

@@ -367,7 +367,7 @@ sharpen:%sigma
 sh:%sigma
 ```
 
-When set, imgproxy will apply the sharpen filter to the resulting image. The valoe of `sigma` defines the size of the mask imgproxy will use.
+When set, imgproxy will apply the sharpen filter to the resulting image. The value of `sigma` defines the size of the mask imgproxy will use.
 
 As an approximate guideline, use 0.5 sigma for 4 pixels/mm (display resolution), 1.0 for 12 pixels/mm and 1.5 for 16 pixels/mm (300 dpi == 12 pixels/mm).