doc/filters: use radius instead of kernel in avgblur

doc/filters.texi

@@ -5431,13 +5431,13 @@ The filter accepts the following options:
 
 @table @option
 @item sizeX
-Set horizontal kernel size.
+Set horizontal radius size.
 
 @item planes
 Set which planes to filter. By default all planes are filtered.
 
 @item sizeY
-Set vertical kernel size, if zero it will be same as @code{sizeX}.
+Set vertical radius size, if zero it will be same as @code{sizeX}.
 Default is @code{0}.
 @end table