doc/filters.texi: Update drawtext docs so that the documented input...

doc/filters.texi: Update drawtext docs so that the documented input width/height variables are compatible with qatar.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/filters.texi

@@ -1088,7 +1088,7 @@ The parameters for @var{x} and @var{y} are expressions containing the
 following constants:
 
 @table @option
-@item w, h
+@item W, H
 the input width and height
 
 @item tw, text_w