doc/filters: add missing '' for blend example

Signed-off-by: Paul B Mahol <onemda@gmail.com>

doc/filters.texi

@@ -5027,7 +5027,7 @@ blend=all_expr='if(gte(T*SH*40+Y,H)*gte((T*40*SW+X)*W/H,W),A,B)'
 @item
 Split diagonally video and shows top and bottom layer on each side:
 @example
-blend=all_expr=if(gt(X,Y*(W/H)),A,B)
+blend=all_expr='if(gt(X,Y*(W/H)),A,B)'
 @end example
 
 @item